Tune and Performance; Symptoms:
FAQ for 7xx/9xx/90 Cars
and B230 Engines:
Performance, Idle Surge and Hunt: Dirty Throttle Body
Idle; TB Cleaned; Now Idle is Too High
Intake Carbon Removal
on Cleaning Up Fuel System Deposits
Dies After Starting, Won't Idle; Needs Intake Cleaning
Idle: Injectors Need Cleaning
Speed Control on Late 7xx/9xx
Start; Frequent Stall or Hesitation: Basic Diagnostics
No Start or No Warm Restart: Radio Suppression, FI Relay, RPM Sensor
Start; Tear in Air Duct Intake Hose
Rich; Black Smoke; Poor Acceleration: ECT, TPS, FI.
Floods and Stalls: ECT?
Start, Dies; Dirty or Faulty IAC
Stalling: Faulty IAC or Hall Sensor
Hesitation on Acceleration; Several Diagnostic Checks
Air Mass Meter Symptoms
Air Box Thermostat and AMM
and Broken Distributor Wires
Failure/No Start: Bad ECU
Poor Idle: ECU Failure with Codes 2-3-1; 2-3-2
Cold Weather Fuel Economy; Bad Engine Knock Sensor
Running; Bad Knock Sensor
Running; Cylinder Diagnosis
Idle; Faulty TPS or ECT
Surge and High Idle: Vacuum Leak
Backfiring While Coasting; TPS Mis-adjusted
Cut-Out While Driving; Electrical Causes
Cuts Out at Speed: Ignition Power Stage Failure
Ignition Shutoff: FI Relay or Ignition Switch
Misfire, Drop in RPM
Driveability Problems with ECU Error Codes; Oxidized Connectors
Runs but Won't Re-Start; Bad RPM Sensor
Stalls When Brakes Applied: Vacuum Leak or FI Relay
Stalls at RPM: FI Relay or Hall Sensor
Running Problems: General Diagnostic Notes
Bad FPR Likely Cause
Stalls During Turn; Bad Fuel Pre-Pump Likely Cause
Performance; Rich Mixture Smell: Diagnostics; Faulty FPR
Perfomance, Bad Acceleration: Faulty FPR
Hesitation: Bad FPR
in Oil: Faulty FPR or Injector
Stalls Repeatedly on Startup: Fuel Pump Check Valve
Stalls, Lights Die: Electrical Ground Fault
Cold Idle Problems -Bad ECT or O2 Sensor and Wiring Harness Notes
Won't Start: Neutral A/T Safety Switch at Fault
Won't Start; Plugged Catalytic Converter
has Poor Acceleration; Diagnostics
Misfires; FI Resistor Pack Defective
Start Problems: Faulty Hall Sensor
Start Problem: Power Stage Overheats
960-Specific Symptoms (see 960 section):
No-Start: Sticking Valves
Stalls: Wiring Harness
Emission Control Problems (High HC, CO, NOx):
Control Problems: High HC, CO or NOx
AMM Air Mass Meter
Engine Coolant Temperature sensor
Control Unit computer (either fuel injection or ignition)
Idle Air Control solenoid valve
Throttle Position Sensor
Vehicle Speed Sensor
Performance, Idle Surge and Hunt: Dirty Throttle Body. My symptoms
are no power to speak of below 2000 rpm. At that point turbo boost occurs
and power comes up, though no where near what it should be. The engine
revs smoothly but runs out of steam before 4500 rpm. I'm left to decide
if the problem is the AMM or the O2 sensor.
[Diagnoses:] Please try cleaning your
throttle body (TB), it can make a world of difference and save you
tons of money. If your B230FT throws oil around like mine, the throttle
body can easily get crudded up and cause problems exactly as you described.
I thought I had a clogged cat, a clogged intercooler, bad O2 sensor, bad
AMM. All these were fine, just a dirty throttle body caused the problem.
[Notes and Tips: illustration copyright
used by permission]
Buy a new TB gasket, disconnect
the throttle switch from the wiring harness, remove the throttle
body from the intake manifold (3 nuts, plus the actuator rod that pops
off once you release the clip. Use a small flat-bladed screwdriver to loosen
the little locking tab on the end of the lower ball-and-socket. The plastic
ends which have the socket & ball attachment on the link rod can break
when they are removed from an old engine, have access to extras first if
you remove the throttle body from the manifold. One right-hand thread,
the other left-hand.) It is a good idea to remove the throttle position
switch/sensor first...don't want to get any carb cleaner in that (but see
the discussions below.) The gasket on my car lifted right off with no fuss,
but if you have to scrape be sure to use a plastic or wood scraper so as
not to damage the aluminum facing. Swab it out with clean rags and lots
of good quality carb cleaner.
[from a Jag owner] The engine breather
is plumbed in upstream of the butterfly. The fumes from the breather are
carrying oil and combustion vapors. When this stuff attempts to past the
butterfly in the mostly closed state, the pressure (and hence temperature)
drop causes the fumes to condense out of the air stream... sticking to
the walls of the throttle housing right around the butterfly... a petroleum
snow storm. As an aside, this buildup can cause really weird problems on
cars with automatic IACs (idle air control: idle speed control). The gunk
will restrict the air flow through the butterfly, forcing the IAC to open
farther to maintain a proper idle speed. On some cars, you end up with
the IAC operating well outside it's designed range... resulting in a major
ring/hunt. Killer surging. Oh, and since the butterfly is plugged, all
of the air, and breather fumes, pass through the IAC. The snowstorm happens
in the IAC, ruining it. Do NOT adjust the throttle stop! Clean the gunk
out. 30K is about right for a XJ40. Every 15K or less on cars without a
heater... BMW, Volvo.
Note on Throttle Body and TPS Adjustments.
For more information on adjusting idle speed on 1989+ LH2.4 cars,
Body and Throttle Position Switch Adjustment.
This expands on the notes below and presents
the official procedure for adjustments.
[More on Routine TB Cleaning, 92 940:]
Try cleaning the throttle body with carb cleaner in a pressurized can.
You need to unclip the throttle linkage from the TB (see tips above regarding
not breaking the clips), unclip the idle/full throttle switch plug, disconnect
the two vacuum lines and the 3 in rubber hose, unscrew the three nuts holding
the TB on. Run carb cleaner through there, clean all varnish off.
Use a toothbrush if needed. Don't spray the cleaner on the TPS.
There are tiny inside vacuum holes that lead to the vacuum tubes that may
be completely plugged. Check the gasket (it will probably be useable once
or twice more) or replace it, and put everything back together. This off
idle bog happens on my 760T every 5-6K miles when the TB gets crudded up.
One of the threads recommended removing
the idle switch assembly when cleaning the throttle body. DO NOT DO THIS
unless you want to mess up the setting and have to readjust it!!
I've cleaned several throttle bodies on LH-Jetronic FI systems dozens of
times and have never had to remove the idle switch or had problems with
it. Just don't soak it in carb cleaner.
But if you can't help yourself -- take
a small screwdriver or other object that will make a fine scratch and scribe
a line across the edge of the throttle sensor switch's flange and it's
mounting bracket, right where the Allen screws hold it tight. Doing so
will get you in the ballpark when it's time for reassembly. After re-assembly,
be sure to check for the "click" when the throttle butterfly comes off
its stop, and adjust as necessary.
[Tips on aligning the TPS on re-installation:]
Check the idle switch on the TB before you do anything...you should hear
a distinct click RIGHT as the throttle is opened signaling the switch is
telling the ECU the engine wants off idle NOW. If this switch is even an
RCH slow, so will your off idle response.
[Tips on reassembly of TB shaft spring:]
I carefully took the spring off the side of the throttle body, noticing
that it was under tension... one full turn, but alas... I forgot to note
if it was under tension one full turn CLOCKWISE or COUNTER CLOCKWISE.
[Response:] The throttle body spring, attached to the throttle body lug
that goes into the throttle body and points out from the throttle body
to the radiator goes CLOCKWISE! The spring has a little bent-out
stop that catches under the idle adjustment screw. You put the spring on
so that catches... twist the dang thing one turn clockwise, put your nut
on, attach the throttle control rod and it's done.
[More on TB and TPS Adjustments from
Gregg Stade] The mechanical stop is a setscrew on the forward side
of the TB aimed up. It has a nut (8 mm, I think) on the underside. It's
right near the big coil spring that turns the throttle to idle. When you
turn the throttle and allow it to return to idle position, you can see
a small arm bear against this setscrew.
The throttle switch is on the backside
of the TB, opposite the setscrew, and is locked by two small screws. It
has an electrical cable plugged into it.
With the TB off the car, I first loosened
the switch so it didn't inadvertently act as a stop (very bad for the switch!).
I backed the setscrew out until the throttle butterfly plate was completely,
jammed shut. Then I turned the setscrew in until it just touched the arm,
and another 1/4 turn. This takes the mechanical force off the butterfly,
so when the throttle slams shut, it doesn't wear the butterfly or damage
the switch. Don't forget to tighten the locknut, and then recheck the adjustment
-- tightening the locknut can change the screw position slightly.
Then, with the throttle at idle position
(the spring forces it there), you carefully turn the switch until it goes
"click". Tighten the two screws. Be sure, as you rotate the throttle (TB
still in your hands) you can clearly hear the switch "click" and that it
does it while the arm is maybe 0.030" off the setscrew.
of Pre-89 TB, TPS and Base Idle:] The p/n's for the gaskets you
will need are 1271488 and 463766. There is an o-ring on the black knob
(pre-'89 only) that you will have to remove to thoroughly clean the housing
p/n 947114. Do not remove the throttle cable, but with a small screwdriver
unclip the small plastic linkage lock from the throttle body and swing
it up , then disconnect the throttle switch . After the housing has been
cleaned the throttle stop is adjusted. Loosen the 8mm lock nut and back
out the screw, turn the screw back in just till it contacts the lever.
Rotate the screw another 90 degrees and lock it down. Install the
throttle switch and rotate it till it clicks with the throttle plate closed.
Lock it down, the switch should make a click as soon as the throttle plate
is moved . Test it several times in your hand at varying opening speeds
make any fine adjustments now. BTW don't loose the o-ring on the switch
if you remove it; it is not available as a spare part. With the housing
back on the car, adjust the linkage rod so that the throttle plate
doesn't move when it is hooked back up. After everything is hooked back
up there are two more things to do. First, adjust the cable with
the threaded piece at the throttle bobbin. You just want a little slack.
Second, base idle has to be reset. (Pre-1989 only:) For this, you will
need a tach/dwell meter. You need to read engine rpm. Warm the engine to
operating temp. Here s the tricky part. There is a blue connector behind
the battery with nothing plugged into it . There should be a blue/white
wire in the connector. Ground this wire it shuts off the idle speed motor
so that base idle can be adjusted with the black knob on the throttle housing.
Base idle for this car should be 700 rpm. When the ground lead is disconnected
the idle should go up to 750rpm's +/- 20 rpm's.
[More on Setting Base Idle Rate:]
Just adjusting the base idle by the set screw is not a good idea. First,
you must check to make sure the throttle housing is clean from oil deposits,
as above. Upon re-assembly, the base idle set screw must be backed
off, then turned in till it just touches the housing. You want to turn
the screw 1/2 turn then lock it down with the 8mm nut. Remount the throttle
position switch and rotate it just so it clicks then lock it down. In 1989,
Volvo did away with the black idle speed screw, everything is controlled
by the fuel ECM .If the throttle plate doesn't return completely and energize
the TP switch, the fuel ECM doesn't know the throttle has returned to idle.
Idle; TB Cleaned; Now Idle is Too High. [Query:] Well my problem
is certainly gone. It now idles at 1600 rpm, but that's a steady 1600.
Did I do something wrong, or was the crud in the throttle body masking
another problem? The barrel of the actuator rod is threaded on both ends,
did I loosen an end accidentally? Should I take it back off and remove
the spring assembly from the side of the body? Since mine had
a very short assembly on the side, is that the idle adjuster? How is it
adjusted? Thanks for any help anyone can provide. [Response: Evan]
Nope, the crud WAS the problem. Crud makes the car idle lousy and slow.
Lazy mechanics simply dial up the idle to mask the problem, rather than
fixing it. You just need to dial the idle back to spec. On the end
of the butterfly shaft, the end where the spring is, there's a stamped
metal plate. It has a 'leg' bent down that rests against the idle stop
screw. The screw is held in place with a lock nut. loosen the nut and adjust
the screw. [See Throttle
Body and Throttle Position Switch Adjustment for more detail on 89+
cars and Adjustment
of Pre-89 TB, TPS and Base Idle: for pre-89] Be careful, the screw
head has a tendency to strip out. In retrospect, you should have
taken a minute to make sure the screw was free while the TB was on the
Another thought: Before you do any of
the above, make sure that the 'leg' on the stamped metal plate actually
touches the stop screw at idle. Some REALLY lazy mechanics just adjust
the throttle cable length at the big obvious pulley, rather than adjust
the stop screw
Intake Carbon Removal. Carbon Removal in Injectors, Intake Manifold,
Valves and Cylinders. Snap-On decoking machine (reported very effective
by Robert Price) The machine is made by Sun (Snap-On owns them). The model
is #EEFS100A and is marketed mainly as an injector cleaner that also removes
some carbon. It does clean injectors but it removes carbon better! Also,
try Motorvac (a variation on the Sun machine.) Other techniques:
Walnut Shell blasting/flushing (many sources
of good success)
As to fuel system cleaners - BG44K is the
heavy duty stuff - and recommended in Volvo and BMW TSB for removing carbon
deposits, particularly from intake valves and fuel injectors. Use no more
often than 2/yr, and only 1/yr if your system is in reasonable condition.
Pour the can in the gas tank only with the tank full. Use it when you will
be able to drive out a tank of gas in a day or two. Plan on changing the
oil and filter soon after use as it can contaminate the oil with unwanted
chemicals. Distributor at website: http://www.bgprod.com
. You can also use a special tool with reservoir and spray attachment hooked
to shop air for injector cleaning. 3 BG chemicals are used, 2 in the sprayer
and one in the tank. Engine is run with chemicals spraying. Takes about
15 minutes. Results were immediate. Idle before treatment was 650 rpm now
850-900. Low speed throttle response is very smooth. These products only
available to professional shops. Find one in your area and try it out-
I highly recommend this product. BG 44K is very effective.
Drive at top speed for 10 minutes (most
GM "Top Engine Cleaner" (very smoky)
Chevron Techron (low risk - sometimes
[Caution from Zippy] Volvo specifically
recommends AGAINST using any fuel or oil additives. I know they used to
suggest it was okay, but then decided that catalytic converter damage is
done when additives are used. Since about 1993 gasoline improvements have
made additives unnecessary.
on Cleaning Up Fuel System Deposits [Mark Burns]
Some of the high end aftermarket fuel
additive packages are very effective at removing deposits in the fuel injectors,
intake ports, intake valves, combustion chambers, and exhaust valves. One
of the best products available is the STP Complete Fuel System Cleaner.
It can out perform Techron and other Techron-based products like Slick
50 and Gumout Regane with respect to intake valve and combustion chamber
clean up. These aftermarket packages can remove any and all deposits
in the system if used at sufficiently high or repetitive dosages although
there is some risk of crankcase lube contamination. All deposits in the
system are carbonaceous. The structure of the deposits in the injectors
is different from the structure of the deposits on the valves is different
from the structure of the deposits on the combustion chamber, but all are
carbon based. The chemistry that is able to remove each type of deposit
depends on the temperature profile that each area sees and the thermal
stability of the detergent in the additive package. Since October
1, 1993, all gasoline marketers are required by the federal government,
according to the Clean Air Act Amendments, to deliver a minimum level of
deposit fighting additives in their fuel. Up to that time, deposit control
additives had been used to differentiate branded gasoline. The major gasoline
marketers usually added at least a competent package to all of their grades.
The lesser known brands rarely added any additives at all. About thirty
percent of the gasoline sold were not additized. The minimum level of deposit
control performance that all gasoline must meet are two standard deposit
control tests-the ASTM D 5500-97 BMW intake-valve deposit test and the
ASTM D 5598-95 Chrysler 2.2-liter port-fuel-injector test using a test
fuel that encompasses the sixty-fifth percentile of U.S. fuel severity
parameters. This federally mandated requirement for gasoline has established
a lowest common denominator for deposit control performance.
Gasoline additive suppliers have found
ways to minimize additive dosage and "beat" these two tests. The result
has been an overall reduction in the deposit control performance of U.S.
gasoline. While a fuel marketer may have data demonstrating that their
specific additive package once passed the BMW intake valve test and the
Chrysler port fuel injection test, there are no guarantees that the gasoline
they are marketing will provide adequate performance in any consumer's
engine. There are many commercial gasoline, available in the market
today, including some major national brands, that, when tested in fleet
test vehicles representing various drivetrain configurations, have developed
more than 1000 milligrams on the intake valves. This is ten times the maximum
amount of deposit allowed for passing the BMW intake valve test. There
is even some evidence that the very low levels of deposit control additives
being used by some gasoline marketers actually make the base gasoline create
greater levels of intake valve deposits in the average engine than the
unadditized gasoline would. It could be suggested that more consumers on
the road today could benefit from the occasional use of a high quality
aftermarket additive package than ever before. In some gasolines on the
market, use of a high quality gas treatment package would certainly be
advised if a driver wants a greater degree of certainty that their engine
will be kept clean.
There are other approaches to the fuel
system cleaning besides adding aftermarket fuel additives. One is to use
water as described in many of the messages posted to this list, one is
the fuel tank/vacuum induction fuel system cleaning approach that many
quick oil change places use and one is the high tech, expensive (some of
these machines cost $4000), and complicated machine approach. Water
is a great deposit remover. It is just like steam cleaning the combustion
chamber. Unfortunately, the heavy components of fuel and fuel additives
are liquid during the combustion process and don't get completely burned.
About 25% of the active fuel additive components (the oligimeric detergent
and fluidizer components) end up in the crankcase. These components may
or may not be compatible with the oil. As you can imagine, water
does not burn. It may leave through the exhaust valve as steam or it may
end up in the crankcase. Do you want 25% of the total amount of water used
in the cleaning process to end up in the crankcase? You can change the
oil right away or you can run the engine long enough and at a sufficient
speed to distill off the water. After any of the serious fuel system cleaning,
the oil should be changes anyway. You can draw your own conclusions about
the effects of water in the crankcase and the prospects of getting all
of it out, but you can clean up the engine just as effectively without
the use of an oil insoluble actor. There are some systems out there that
use water in the fuel system cleaning. I think these systems usually employ
some kind of very expensive machine. I don't think they clean more effectively
than the fuel tank/vacuum induction fuel system cleaner. Cars are sensitive
to deposits, but not that sensitive.
Tank/Vacuum System Cleaners
The fuel tank/vacuum induction fuel system
cleaner cleans injectors, intake valves and the combustion chambers through
the action of the bottle of additive poured into the fuel tank. The vacuum
inducted intake system cleaner is added through a vacuum line behind the
throttle plate. The purpose of the intake system cleaning is to remove
deposits left by the PCV and the EGR as well as aiding in cleaning up the
intake valves, ports, and combustion chambers. One brand that is very effective
and provides a high quality product is C.A.T. Products makers of Run Rite.
The tool used to induct the intake system
cleaner into the vacuum line is usually a metal bottle with a tube in it
that connects to a hose with a fitting on one end to connect to a vacuum
line close to the throttle plate on the vacuum side. There will be no problem
as long as the engine is running, it will suck in the cleaner. If the engine
stops but the fluid keeps flowing, you can hydrolock the engine and damage
valves, rods, pistons and gaskets. These tools often have a valve in line
and a clear portion in the hose after the valve to adjust the feed and
monitor the flow so that it is a steady drip. The fluid usually used in
the bottle is an air intake/throttle plate cleaner package. Only the additized
fuel in the tank goes through the injectors. Based on what I have seen,
this should work as well and any injector cleaning scheme on the market.
Fuel injectors deposits are not as much
of a problem now as they were a few years ago. New injectors are more resistant
to deposits and most gasolines, as poor as they are at controlling most
deposits today, still can keep injectors (and carburetors) clean. STP Fuel
System Cleaner works very, very well. Two bottles should have them spotless
and will clean the valves, ports, and the combustion chambers. The
fuel tank/vacuum induction fuel system cleaning makes an immediate difference
in the way the car runs. It must have something to do with the EGR and
PVC deposits. You can also try to replace the PCV. The beauty of
the vacuum induction fuel system cleaner approach is that it doesn't require
a degree in mechanical engineering and a master mechanic certification
to operate: pour a bottle in the tank then find a vacuum hose and suck
a bottle of the intake system cleaner into the intake. I don't think the
systems that utilize the expensive machines actually clean the fuel system
any more thoroughly.
The problem with the machine systems hooked
up to the fuel rails is that they can not clean the parts of the system
that the fuel does not get to. Cleaning through port fuel injectors
can clean the injectors, intake ports, intake valves, and combustion chambers.
Cleaning through the vacuum line cleans the entire intake manifold,
intake ports, intake valves and combustion chambers while the fuel additive
added to the tank cleans the injectors, intake ports, intake valves, and
combustion chambers, albeit at a slower rate as the fuel in the tank is
burned over about 350 miles. I think the fuel tank/vacuum induction fuel
system cleaner approach may, in fact, provide a more thorough cleaning.
I personally do not believe that the expensive
and complicated machine/high pressure systems have any advantage over the
simple approach that we are using. They do, however, have a major drawback
in that there are more things to go wrong. The technician has to disconnect
the fuel pump and connect to the fuel rail. There is big potential for
disaster with this approach if the technician is not highly trained. It
is pretty easy to pull off a vacuum line and suck in the cleaning solvent.
If the vacuum line is not reconnected properly, the car will not run, but
it is easy to diagnose and fix. It is also unlikely to burn the car up
if the technician doesn't do something right.
The walnut shell blasting can be done
without removing the head. It is a fairly difficult operation and requires
the right equipment. You also have to make sure you remove all of the residual
walnut shell. All in all, the aftermarket fuel additive packages or the
fuel tank/vacuum induction fuel system cleaning are probably the least
Dies After Starting, Won't Idle; Needs Intake Cleaning. My '85 with
230F had a similar problem. It would die after starting and put into gear
when cold and Idle was not stable. Here's what I did to fix it:
At this point it was running better but
Replaced all vacuum hoses and checked
for vacuum leaks.
Removed and cleaned air control valve.
Removed and cleaned throttle body in fuel
carb cleaner, replaced all gaskets.
Car now runs very strong.
Adjusted throttle body linkage and throttle
position sensor per the book. It has to be done in sequence.
Ran a can of BG44K through the tank to
clean up intake valve and fuel system deposits.
Idle: Injectors Need Cleaning. See Injector
Cleaning for more information on how to solve chronic idle problems
due to dirty injectors.
Speed Control on Late 7xx/9xx. On late model cars such as yours,
the idle speed is controlled by a bypass system. When your foot is
off the gas, the throttle plate is fully closed and a separate idle control
valve admits the required amount of air to get the engine to idle at 750
rpm. This separate valve is controlled by the computer. No
adjustments are possible. It sounds like the linkage to your throttle
plate may be out of adjustment. I have a 1990 Turbo model and I think
yours is very similar. I have adjusted the linkage on mine, but I
used a factory service manual while I did it.
Start; Frequent Stall or Hesitation: Basic Diagnostics. [Query:]
I have an 88 740 non turbo 150k miles. Intermittently the engine will stall
typically at lower speeds or idle. It will not restart unless I turn off
the ignition and back on again as if this resets something (the computer?).
Also there is a stumble or miss when starting out from a stop. This is
fairly consistent. Another symptom is that intermittently at cruising
speed of say 60mph the engine appears to cut out for only a half second,
this will continue several times a minute until I shut off the engine and
restart. Plugs and wires good, throttle body clean, fuel pressure
OK, O2 sensor OK, have checked all connectors that are easy to get at.
It appears that the computer sometimes gets out of whack for some reason
and wants to be reset. Any suggestions?
[Basic Diagnostic and Preventive Maintenance
Checklist from Paul Grimshaw]
You've either got a fuel or spark problem.
Here is a basic diagnostic checklist:
1. Confirm condition of distributor
cap, rotor, spark plug wires and spark plugs. If *all* of these have
not been replaced in the past two years with OEM or better (ie. higher
quality ~ more expensive), do so now. (Yeah, I know that you have
not yet determined that the problem is spark related, but there's no sense
troubleshooting a problem unless the basic bits of maintenance are completed
and certified correct).
2. Remove each spark plug, ground the threaded
base against the engine block and, with the spark plug wire connected to
*only that plug*, have a friend crank the engine. You should see
a spark. If not, there is a problem with spark delivery. Repeat
with each plug/wire. Note: In most cases, its not the spark but the
fuel. That said, any failure to complete basic maintenance on your
engine will only lead to a poor running engine and multiple, intermittent
3. If your have not replaced the fuel filter
in the past 3 years, do so now. The fuel filter/fuel pump is located
under the passenger footwell (driver's side). Replacing the filter
is not a particularly difficult job for the experienced shade tree mechanic
with a complete set of hand tools. If you're unsure of your experience
level or tool crib, have the job done professionally. Ensure that
the sub-assembly is completely cleaned and dried before disassembly as
FI systems do not react well to ingested dirt. It will require Volvo
PN 1389562-8. While you're replacing filters, don't forget the air
filter -- you engine will always run better with a new one of these!
Again, a bit of basic maintenance that should never be skipped.
4. If you lack a fuel pressure gauge capable
of reading fuel pressure to at least 400 kpa and lacking the proper fittings,
true troubleshooting will be difficult. Your car's LH system is supposed
to operate at a constant 300 kpa.
5. Lacking the proper diagnostic equipment,
your only solution will be to replace each component until the problem
is solved. I won't list all of the possible problem components as
most situations are traced back to either the main fuel pump or the pressure
regulator. Use a long piece of hose to listen for the main fuel pump
as the car is being cranked. If you cannot hear the fuel pump turn,
check the voltage using a VOM.
6. If all is well, skip to the fuel
pressure regulator. Replace the unit with Volvo PN 1389564-4,
but may also be replaced by Bendix PN 4088942-0001.
7. If the car now runs, but frequently
requires long periods of cranking to start, then suspect the check valve
in the fuel pump. The check valve is a threaded in-line valve that
maintains line pressure between 200-300 kpa after the engine is shut off.
8. If the car still does not start,
check the fuel
pump relay. If in doubt, replace it with Volvo PN 3523608-3.
Hope this helps. Remember the merits
of maintenance before trying to solve any recurring problem.
9. Conclusion. The beauty in the
approach listed is that you first complete the basic maintenance required;
car's just won't run well without regular maintenance! Second, you're
using a rudimentary tool (the hose) to check for fuel pump operation.
That's important since the fuel pump is a very expensive piece of gear
that you would not really wish to replace unless it is dead. Third,
there's some merit in repair by replacement, especially if you lack the
proper fuel system diagnostic equipment. The pressure regulator is
about $40 buck and, with age, will eventually die anyway. So replacing
it just saves you being stranded at some point in time. Finally,
the pump relay is the next most likely culprit. [Editor's Note: See
the following notes about rpm or Hall sensor and radio suppression relays;
these can also be frequent culprits.]
[Response: Boris] Do the spark plug test
next time you have this failure. If you have a spark, then a fuel
injection relay is probably the culprit.
[Response: Mark Klein] Sounds like
you've been fairly thorough already. There is a radio
noise suppression relay on the coolant reservoir which can go bad and
cause a variety of similar symptoms including not running at all.
Be sure the fuel
pump relay is in good shape. It is the white one in the 2nd row back
on the far left. Check the date code printed on the side. If it is the
original, it wouldn't hurt to replace it anyway. The Hall
sensor sender wiring (pre-89 cars) coming out of the bottom of the
distributor can short out against the distributor housing if the plastic
connector breaks. This is quite common but I doubt if this is giving
your symptoms. The FI
control unit itself can go bad. One of the more common circuits which
fails is the one that grounds the fuel pump relay and, in turn, turns the
fuel pump on.
I doubt if an air mass meter would give
the symptoms you're getting but you might try removing and reconnecting
the electrical connector a few times.
Intermittent No Start or No Warm Restart:
Radio Suppression, FI Relay, RPM Sensor. [Query:] Sometimes the
’88 749GL just won't start. It seems like it's not getting gas when this
happens, but after sitting for awhile (a few hours, overnight, or occasionally
just a few minutes) it starts right up like nothing is wrong [Editor:]
If your car fails to start until after it has cooled down, the three items
to check are the rpm sensor,
suppression relay and fuel
injection relay solder joints. [Response 1:] Try the fuel injection
relay. Find the relay in your center console relay bank and tap it to see
if this restores fuel flow (starting immediately). [Response 2:]
I have a 90 740gl that had the same problem. When you're cranking the engine
and it won't start, is the tachometer needle moving a little? If not, as
mine didn't, I believe you want to check into the RPM sensor. Check the
RPM sensor located on the back of your engine, connected to the bell housing.
The wire runs up the back of the engine compartment towards the drivers
side. Look for the part number on the wire and check if the part number
ends with 399. This rpm sensor has a heat related problem: common for it
to cause a no- start but able to start a little while later. Since I replaced
mine for about 30$ I haven’t had the problem. [Editor: post-88 cars have
this sensor; pre-88 cars have a Hall
sensor inside the distributor.] See also the Radio
Start; Tear in Air Duct Intake Hose. [Query: David Smith, courtesy
UK Volvo Club] For the last six months it has been getting harder to start.
Up to now there has been no problem - has 170,000 miles. Fuses are OK and
I replaced the distributor cap, rotor, installed a new wire set and replaced
the spark plugs. I also noticed that one of the wires leading to the coil
was also in very bad shape at the coil end. I snipped off some of the wire
and replaced the female connector. I’ve been to two dealers who put it
on the computer ‘scope’ - no problem shows up. One dealer cleaned the throttle
body (dirty) and the other installed a new crank case sensor. A third dealer
said it may be a poor ground (all seem OK) or a computer module. I said
that nothing showed up on the computer analysis at two different dealers
- and he said that it probably wasn’t the module in that case. Very recently
it just wouldn’t start. It turned over OK and I unsuccessfully tried to
jump start it from two different vehicles, a light truck and a V8 Chevy
(using a good jumper set). There was spark from the plugs when turning
over. We also sprayed di-ethyl ether in to the engine via the fuel injection
system and engine fired OK when turned over. Tried again - turning over
OK - but not starting. However, just three hours later it started up with
no problem on its own and has run OK for 3 days. From what I can gather
I suspect that it may be an ECU problem - but nothing has shown up on the
computer scope at two dealers. [Later reply from David:] A
local mechanic found a small tear in the large diameter air hose (has the
bellows) that runs from the air mass sensor (and air filter end) up to
the throttle body. He did a quick repair using a sealant. Since then everything
has been fine - no more starting problems. It's very easy to inspect this
air hose - just undo the clamps at either end - make sure they are tight
when you put the hose back on! I spoke to another Volvo dealer recently
- regarded as the best in Toronto - and they said that this air hose vibrates
- hence the need for the bellows section. In addition the material used
to make the hose does start to perish over time and this may lead to some
brittleness and tears appearing. When that happens - it can lead to all
sorts of problems they claimed. The replacement hose costs just $50 Canadian.
Rich; Black Smoke; Poor Acceleration: ECT, TPS, FI. [Query:]
I have a 1985 740t with 244,000 miles and a m47 manual transmission. It
has the 230ft engine that is all original including the non-liquid turbo
without an overhaul. The car runs rough throughout the rpm range and put
out black smoke while doing so. Lots of black smoke. Other times (less
and less) it runs like a top, no problem. I replaced the fuel pressure
regulator but it did not help. I am getting about 43psi at idle. When it
is running in the bad mode the fuel pressure is still 43psi. When I drive
the car I have to accelerate either very easy or in wide open throttle
to get the car to go. It is at its worst just as boost comes on. Any Ideas?
[Response 1:] Here's one idea, but it really applies to the non-turbo engine
-- so I could be a million miles off base on yours....... The fuel
mixture is influenced by the block temperature sensor (Engine Coolant Temperature
ECT see Diagnosing
ECT Failures ). A cold engine requires more fuel, and a warm
engine needs less fuel. A cold sensor is a high electrical resistance.
An intermittent open condition of the sensor, the connections, or the wiring
harness will "trick" the FI ecu into thinking it's about -60 degrees. It
will pump in tons of fuel. On many of the engines -- and again, I'm
not positive about yours -- there are two sensors. The sensor for the dash
gauge is in the head, about under the intake runner for cylinder #2. The
sensor for the FI system is in the head, about under the intake runner
for cylinder #3. Tough to get to.
I helped fix a situation (on a '90 740,
non-turbo, Rex-Regina system -- yours is Bosch) where the connector had
pushed out of the plastic housing but made intermittent contact with the
spade connector in the sensor. Some days, it ran great. Other days it flooded
so bad it wouldn't always start. Fixing that stupid problem made
a world of difference!
[Response 2:] Also..... be sure
the throttle switch (Throttle Position Sensor TPS) is working and adjusted,
and the AMM is operational.
[Response 3:] Very similar situation in
my '86 745T drove me nearly crazy for about 2 months. It was a leaking
Floods and Stalls: ECT?. [Query:] I have a 1986 volvo 760 turbo
with 207,000 miles on it. I recently purchased and at the time it was running
on three cylinders and had sat for about six months. I replaced the flux
amplifier and it ran on all cylinders. The man who sold it to me also told
me that he had an intermittent problem... It would stall out occasionally.
He was true to his word...If it is cold (running about 15 min. ) and you
give it too much gas it starts to flood out. If you floor it while it is
flooding out it starts to catch on the other cylinders and eventually will
go. If you stop the car while it is flooding out it will either stall or
run on 1 or 2 cylinders. One time I disconnected the map sensor while it
was flooding out and it started to idle normally. While driving behind
the Volvo it spits black smoke when it is flooding and sometimes even when
it isn't acting up a little bit of black smoke will come out of the exhaust.
The problem clears up if you drive about 30 miles on the highway at constant
speed. After that you can stop it idles pretty good.... misfires a tiny
bit.... and you can take off like a bat out of hell...also about a week
ago I tried to start the car and it would only run on 1 cylinder, then
not at all. I pulled the plugs and they were all fouled I put in new plugs
and it fired right up. I tested to make sure every cylinder was firing
and they were...is it my computer?? map sensor???
oxygen sensor??? (the O2 sensor must be
changed every 30,000 but will it make the car flood out??? or the distributor,,,
which seems pretty clean.
[Response: Don Foster] I'd consider looking
at the temperature sensor or connections to it. There are two sensors --
one for the temp gauge, one for the FI ecu. The gauge sensor is a one-wire
device, the FI temp sensor is a two-wire device. The FI temp sensor
is mounted in the head, under the intake manifold, approximately under
runner #3. It's not impossible to get to -- just almost. (The gauge sensor
is under runner #2 -- ignore it.)
The sensor is an NTC thermistor -- that's
"negative temperature coefficient", or as the temperature drops, the resistance
rises. If the sensor fails or if you have a bad, broken, or corroded connection
at the sensor (or anywhere in the harness going to it) the FI ecu measures
high or infinite resistance. The ecu thinks it's about -100 degrees, and
sends tons more fuel to the engine. And if it's an intermittent
problem, your engine can be running fine and then go into gas overload
5 seconds later. And vice-versa. I helped fix a problem in
a '90 non-turbo with terrible intermittent flooding. We found one connector
in the sensor plug had "loosened" so when the plug was pushed onto the
sensor, the connector got pushed back up the plastic housing and sorta
dangled freely -- one second it touched, the next it didn't. It took hours
to zero in on this tiny failing. See more at Diagnosing
Start, Dies; Dirty or Faulty IAC. [Symptoms:] Cold Start: Car starts
immediately with no throttle application, idles smoothly for about one
second, then hesitates and dies. Ditto for my 1990 245DL. Starts up cold
for one second, then dies. Sometimes, all cranking I wish to do will not
restart it. If I sit and wait about 5 minutes, it will start and run like
a kitten, with no problems. I have replaced all spark plugs, s.p cables,
rotor, cap. If it is a computer problem, can I fix it? [Suggestions:]
Check the IAC (idle air control- This provides for more air, or faster
idle on start up. The fact that it starts readily when cold indicates the
cold start injector is working properly. See hints below.
[Symptom:] Poor idle. [Diagnosis:]
Clean the dirty Idle Air Control Valve. I should have remembered. The car
did not start at ALL. So, I took out, removed the two hoses and cleaned
it by spraying it with engine grease remover. It had a lot of dirt in it.
The Idle Air Control Valve is located under the intake manifold; around
the second cylinder. It is about 3 inches long and about 1.5 inches in
diameter (I do not have one next to me so these dimensions are from my
memory.) It has two rubber hoses connected to it through a "T" connection.
It also has a snap-on electrical plug. This is most likely your problem.
Here is how I clean it:
If there is no improvement, repeat above process
3 or 4 times. However, if you see measured improvement, just continue cleaning
it until you are satisfied. If you still have problems, then either your
IAC is dead and must be replaced or the problem is somewhere else. [Editor's
Note: See the IAC rebuilding procedure noted below.]
Take IAC off the car.
Remove all hoses.
Spray some engine degreaser/cleaner into
the opening end which is opposite to the electric connector end and shake
it around. Make sure you plug the other end with your palm to trap the
liquid inside the IAC.
Open and close the flapper valve with
a small screw driver to loosen any dirt, etc. contained inside.
Turn upside down the IAC to empty out
Repeat step 3 through 5 about 8 times.
Test the IAC on the car without permanently
installing it. You might have to repositioned some hosing to accomplish
Poor/Surging Idle: Idle Speed Control
Motor. [Tip from Anonymous Source]
I have a 1987 model 780 with about 115K
miles. Just wanted to share my experiences:
About 10% of the time my Idle would start
to surge between 200 and 1200 rpm while sitting at a light with the foot
on the brake. This up and down action would load up the engine with
fuel and it would start emitting black smoke. It seemed to be somewhat
related to engine temperature. Occasionally it would get bad enough
to kill the engine, but at highway speeds it seemed to run fine.
I first looked at the idle speed motor, but it appeared to be functioning
properly when the voltage was applied to the terminals per the normal checks.
I then performed the disassembly and cleaning of the idle speed motor as
described in this web site. Even though the inside of the motor was reasonable
clean, I went ahead and polished the commutator. I detected that
the bearings that the rotor turns on were a bit gummy so I cleaned it well
and lubricated it with a bit of graphite. Ever since this little
operation it has been idling like a champ. I suspect that small bits of
commutator crud and wear on the brushes may have been creating dead spots
and poor response to idle demands form the processor.
Stalling: Faulty IAC or Hall Sensor. [Symptom:] Intermittent stalling/no
start condition on an 86 740 (b230F) manual trans: occasionally dying (engine
suddenly has absolutely no power, but the dash lights do not come on) while
driving on the expressway and around town. After pulling over to the side,
the car invariably will restart and seem to run fine. No rough idle or
other problems were noted.
[Fixes Attempted:] Several months back
(per list advice), I replaced the fuel relay, which up until the last week
seems to have cured the problem. Over the last couple of days, the car
has started acting up again in the same manner. Now, the car also, when
it does restart, sputters and runs extremely poor for awhile, then goes
back to normal. Also, in the driveway, I was able to get the car to reliably
stall when he put on the brakes. I replaced the idle speed motor, which
seems to have affected the brake-induced stalling (plus the car idles much
better), but not the intermittent stalling. He also noticed a bare temp-sending
wire, which he cleaned and re-taped (but we don't suspect that has to do
with this failure.) Per past list advice, I've suggested
[Diagnostics and Suggested Fixes:]
Below you'll find two procedures [Composite from Steve
examining all vacuum lines
rechecking fuel-pump relay and socket
for cold-solder joint/overheating socket
in-tank pump and screen
the power stage connectors for corrosion
radio suppression relay
general condition of wiring harness (the
car lives in the very heavily salted road conditions of northern Ohio)
McChesney and others]. Hope
they will help.
[Fix 1: Idle Air Control "IAC" Valve
Rebuild] Save $150 by fixing IAC instead of replacing it. Symptoms:
Solution: clean the slip ring inside
the IAC (note: the IAC on newer LH cannot be dissassembled.)
The IAC is a metal can beneath the intake manifold, with two 1" hoses that
feed extra intake air around the throttle body to control idle RPM, and
reacts to loads like A/C or heavy electrical (alternator) loads.
The "valve" has three pins, the center is a constant 12V supply, and the
outside pins are pulsed by the ECM to ground, to either open or close a
circular shutter -- looks something like a revolving door. (Except
mind would only open, and close halfway.)
No fast idle at start up.
This is a common problem on cars that use
this Bosch system, including Volvos and BMWs. It's especially bad
on turbo cars, and cars that use crankcase air for idle makeup because
of the oil that comes along with the air.
Grounding CIS (test point) does not have
any effect on idle. (grounding the test point should disable air control
OR: The idle ('87 745ti) shoots
up to about 2500 RPM. No amount of cooling off, throttle blipping,
or general search for vacuum leaks would bring it down. I checked
all the hoses, fuel pressure, mass air sensor, and coolant temp sensor.
Everything was fine. And the car ran fine too, just a high idle.
Start the engine.
[Another Tale of IAC Repair:] The following
Idle Air Control Valve repair might be of interest to fellow tightwads
who experience a HIGH idle problem. I removed the bad IAC. The valve was
not frozen, but moved quite easily back and forth. That's when I decided
to open it up and look inside. Opening involved prying open several tabs
around the unit's waist. Out came the solenoid innards which looks like
the armature of a small motor with enamel wire winding around three arms.
The cap removed from the valve contains a magnetic lining along the wall
and three brush contacts near the top. The top of the armature where the
brushes should contact was VERY greasy. I cleaned that and the contact
brushes inside the IAC cap. A multimeter confirmed that there was continuity
among all three arms of the armature: ohms1to2=21, ohms2to3=21, ohms1to3=42.
So apart from the grease, I could see no problems. Therefore, I put the
armature back in the IAC cap. This required some trial and error because
the armature keeps wanting to stick to the magnetic sides of the cap. Once
back together, I bent the holding tabs back in place (keeping pressure
between the two parts).
Disconnect quick connector at air control
Test for battery voltage on the middle
pin (it is pin number 2 "GREEN wire" if you slide the rubber off), you
should detect 12V or so (with engine running).
Turn off the engine.
Test for ohms on IAC between middle pin
and any side ones, you should have between 6 and 20 ohms in each case depending
on the model. If you do have this resistance, then it means that the motor
of the IAC is good. If not, don't give up yet.
You can pull the valve easily if you disconnect
the hoses from the manifold and the intake hose, disconnect the 3-pin electrical
connector, and loosen the top nut on the band clamp around the valve body.
Off the car, look down the bore of the valve, and then by supplying the
center pin with 12V and shorting either of the outside pins to ground,
the "door" should fully open or fully close.
Now, you are going to clean the slip ring
(the one wherein the brushes are touching) inside the IAC. Take the IAC
out and take it apart. [Editor's note: this does not appear possible on
newer IACs with no housing clips.) It's easy. But, before you take it apart,
mark the housing (a pen mark or screwdriver will do) so you can assemble
it in the same position. IT IS IMPORTANT!
With a small screwdriver pry up the clips
or crimps which are holding the housing (there are four of them). Once
they are up, pry on the housing a bit and the whole thing should come out.
At this point, you will encounter a bit of resistance because of the strong
magnet inside, but there is nothing to worry about, just keep going until
everything comes out. Be careful not to lose the o-ring, or the small cone
piece -- which will probably fall off the armature shaft and be laying
at the bottom of the barrel with the permanent magnets and the wiper contacts.
You MUST NOT try to push it in once you started separating the housing
(or you will destroy the brushes inside the valve). Once you open it, you
will understand why.
You will then see oil (especially if its
a turbo car) and various crud built-up all over the armature, the commutator,
and in between the commutator segments - where I'm pretty sure it shorts
some of the windings together. Clean it up and flush away all crud
carefully with brake cleaner.
Now, clean the contact points with very
fine sandpaper or a Scotchbrite pad and polish them. Be careful; don't
bend anything. Again, make sure you don't lose the little cone piece.
You'll need to get it out of the barrel for re-assembly - it will come
out easily but you have to use something non-metallic because of the strong
magnets around the housing.
Putting it back is easy but tricky. When
everything was spotless and oil free, put the cone on the end of the armature
shaft, so that it tapers away from the armature. The cone is important,
because it acts as a ramp when you replace the armature, gradually spreading
the wipers so they land on the commutator without bending them out of contact.
Also, a little silicone grease on the o-ring seal can't hurt. Once you
take it out, slide it over the shaft and keep it in the upward position
(so it won't fall off) while assembling it.
To re-assemble, put the tee upside-down
on the workbench, with the cone in place, and gradually feed the barrel
down. It's a bit of a trick to keep it straight, because the permanent
magnets have a habit of sucking the armature from side-to-side. It
doesn't take much force. Once the armature is seated properly, re-align
the marks on the barrel and tee by simply twisting the tee relative to
the barrel. If you forget this step, the valve may likely not open
or close fully. After doing so, then you are done. Push back
the clips (if you have clips, and don't use a hammer)
Re-test the valve as above with test leads
before the final crimping. Mine worked fine now, fully open, fully
closed. I then squeezed the crimp tabs back into place with large
channelock-type pliers, and double-checked the alignment marks. Replace
the valve, making sure the flow arrow on the tee points in the proper direction.
The arrow should point TO the intake, FROM the main throttle inlet hose.
Tighten the hose clamps, and viola! "New" IAC valve. The entire procedure,
including the pulling of the valve out and installing it back takes about
30--45 minutes. Test the side pins for ohms and re-install the IAC. You
should now have a fast idle. Oh, make sure your PRM at idle is about 800
rpm. PS: I drilled and put in a small screw to prevent housing from rotating.
The first time I started the car after
installing the cleaned IAC, the idle was still high. However, I removed
the IAC again, wiggled the valve back and forth a few times, and reinstalled
the IAC. I checked the resistances at the electrical connection pins and
this checked out. Maybe the contact brushes hadn't quite seated themselves
yet. The second start resulted in a nice idle control. In PARK, the car
idled 600-700 RPM when warm as opposed to about 1600 RPM when the IAC didn't
work. I've been driving with this cleaned IAC for several days now. The
idle system is working fine. And I hope it stays that way! Was all this
trouble worth it? Well for a tightwad, I'd have to say yes (since the IAC
was successfully repaired).
[Fix 2: Check
Hall Sensor] Below is the procedure to check Hall Sensor (inside distributor)
for B230F/FT ignition systems: EZ-117K, EZ-118K on pre-89 cars.
Disconnect WHITE/RED AND BLUE leads from
ignition coil (to prevent arcing damage).
Undo the distributor connector
When the ignition is ON the Voltage between
positive terminal (red lead -Nr. 3) and ground should be approx. 11V. Voltage
between (blue lead - Nr.2; middle one) and ground should be 5V. Resistance
between (black lead - Nr. 1) and ground should be "0" Ohms-
Undo connector from the control unit (above
the brake pedal) and remove the sealing washers (plastic inserts on the
side of the connector). Replace connector without cover or sealing washers.
Disconnect white/red and blue leads from
Measure voltage between terminal 24 at
rear of connector (blue lead) and ground. NOTE: Connector must be attached
to control unit.
Switch on ignition. Turn crankshaft by
hand. Voltage should indicate OVER 1.8V each time a vane passes Hall generator.
Instrument should read approx. "0" (0 - 0.7V) each time an opening passes
Hall generator. Correct voltage: Less then 0.7V or more than 1.8V.
Hesitation on Acceleration; Several Diagnostic Checks. [Symptom:] Slight
hesitation on acceleration.
[Diagnosis 1:] Check the health of the
knock sensor. It is an inexpensive part that "listens" to the engine, senses
knock and automatically retards the timing. They get full of crud etc.
and give false information. Incorrectly retarded timing will give you a
[Diagnosis 2:] A "slightly" faulty AMM,
i.e. during low air mass conditions, can output a too low "air mass" signal
to the ECU causing a too narrow (lean) basic injector ON pulse width. Carefully
check the AMM's wiring harness plug. Slight misalignment of the female
contacts can cause ECU input problems.
[Diagnosis 3:] Clean throttle body.
[Diagnosis 4:] Is your temperature gauge
correct? Is the engine heating up properly? I had the exact same problem
and it was finally "cured" several weeks ago after the dealer replaced
the thermostat that controls the temperature gauge. Now the engine is a
lot more smoother and the car has regained some of it's power!
[Diagnosis 5:] The fuel pressure regulator
is worth a careful inspection. The fuel pressure must rise instantly in
response to the vacuum signal fall that accompanies a throttle opening.
A hardened diaphragm might be causing the fuel pressure that has been lessened
by the fuel pressure regulator to not increase as rapidly as it must and
you won't get the appropriate fuel quantity in spite of lengthened injector
duration. Try an acceleration test with the vacuum hose pulled off fuel
pressure regulator. Easiest way to check the FPR is to pull the vacuum
hose off of it while the engine is idling. If the idle picks up, your FPR
Air Mass Meter Symptoms. [Would someone give me give me a description
of what a bad AMM feels like?] My single experience with a bad AMM was
that the engine would barely run at all and was not driveable. I think
it reverts to "limp home mode" if you pull the connector off. But I still
think you should clean the throttle body. Since the purpose of an AMM is
to regulate air flow to the throttle body, it usually sits right on top
of the air cleaner or close by and there is some sort of connector on it.
Remove the connector and see how your engine runs. It should run like crap,
i.e. no power, idle fluctuates wildly, or engine stalls. If this is similar
to the problems you're experiencing now, chances are that the AMM is bad.
Another thing to check is that there aren't any air leaks between your
AMM and the throttle body.
More than one inexperienced technician
has failed to cure an intermittent driveability problem by replacing the
mass airflow sensor. Even if your pinpoint tests suggest a failing sensor,
always inspect the sensor's harness connector for loose terminals first.
Be sure all the terminals are locked securely inside the connector before
you condemn the mass airflow sensor. Some routinely remove a harness connector
and then reinstall it to see if "reseating" the connector solves the car's
problem. But reseating the connector on this Bosch airflow sensor may complicate
diagnosis by pushing the loose terminal outward. So, your quick-fix trick
actually makes the car run worse than before!
Air Box Thermostat and AMM. Unless you live in a very cold climate
I don't recommend replacing the cold/hot intake air valve and plumbing.
The valve has a tendency to fail in the hot air position thus supplying
your turbo with pre heated air and having a tendency to fry your air mass
meter. In addition it restricts the incoming air flow as well. Check out
the Turbobricks net page ( http://www.turbobricks.org/).
There are instructions there on doing an "air box mod" that recommends
and explains how to do away with the hot/cold air valve and plumbing and
redirect the incoming air to the air box.
Though I don't know why the thermostat
isn't available for 740 but it is for 240. In the case of 740 they want
you to buy the complete air duct w/ thermostat. I bought a thermostat for
240 and fitted in my 740 and I checked its operation. It works perfect.
The air flap is closed at +5C (41F), +10C (50F) half open, +15C (59F) only
[Editor's Note: See Air
Box Thermostat Change for more information about changing the thermostat
in 7xx/9xx cars.
and Broken Distributor Wires. [Symptoms: Misfire is very pronounced
under heavy throttle -- think of a mechanical bull -- which leads me to
suspect something in boost control. But the tach also shows about 150 RPM
variation at idle, which hints that the misfire exists under no load conditions.]
I had this happen to my 85 as well. It turned out to be the connector for
the distributor wires under the dist. cap. had broken, and the wire insulation
had disintegrated. The wires were grounding out on the distributor housing.
For a while it would run fine, then it would miss like you say. I went
around the barn on this and spent significant time on things that made
minimal difference. I don't know if I broke the connector off or not, but
the wires had fallen to the insulation eaters that befall Volvos of that
era. I ended up putting in a new distributor because it was convenient
and the car had completely died. I did find out that a new hall effect
pickup can be installed by drilling out the rivets on the old one and screwing
on a new one. To check, put your hand under the bottom of the distributor
and see if there is significant wiggle in the connector there. The cap
should hold it tight, but on mine, once I was told where to look, it was
obvious what the problem was.
Failure/No Start: Bad ECU [This note is from Car Electronic Service
in the UK at http://www.carelect.demon.co.uk/index.html
]Applies to: 1988 to 1992 740 and 740 GLE with Bosch Jetronic LH 2.4
fuel injection ECUs part numbers 0280 000 590/591/594/595. Symptoms...No
prior warning, car usually breaks down on road or fails to start when warm
(loss of fuel pump control).
Mechanic diagnoses fuel injection ECU
failure - loss of fuel pump relay control.
ACCURATELY Identify fuel pump relay and
remove. Connect jumper lead to provide 12 volt feed to fuel pump after
ACCURATELY identifying relay socket pins.
If car now starts and runs, take for road
test to confirm OK (put on exhaust gas analyser first with catalyst equipped
vehicles). IT IS NOT OK to use car with fixed feed to fuel pump.
If OK, send ECU to Car Electronic Services
fitting of our own design fuel pump relay control circuit.
If car doesn't run with jumper lead fitted,
then ECU is probably beyond hope as not normally repairable.
Poor Idle: ECU Failure with Codes 2-3-1; 2-3-2. [Tip from Al
Nettleton] To summarize my original message (they don't always stay around
long enough to pick up the thread for a status update):
1. Error codes 2-3-2 & 2-3-1 (Mixture too lean or rich)
2. O2 sensor replaced with no success
3. Volvo mechanics recommend ECU replacement & I posted the problem
on the Brick board, describing specific additional symptoms
a. If the error codes were read &
cleared, they come back almost immediately
b. If the ECU is unplugged (or the battery
disconnected), the codes clear but come back after about 150 miles. For
the first 50, or so, of these, idle is rough, next 50 - hesitation from
a standing stop, the last 50 - runs well. BUT . . .
c. Running lean.
Thanks to Steve, Craig & Allison for
making recommendations to check for vacuum leaks, clean throttle body,
dielectric grease on sensor connectors (except O2), etc.
Successful repair required replacing the
air mass meter (AMM), intake bellows between AMM and throttle body, and
ECU. Parts swapping confirmed the need for all three. One important
finding - the air bellows can leak at the end connections, even though
there are no cracks or holes in the bellows and the clamps are tight. Apparently
they get to be so inflexible that a good seal is not possible.
We should now be ready for another 100K.
Cold Weather Fuel Economy; Bad Engine Knock Sensor. One suggestion,
replace your knock sensor. I did it on mine and I notice the difference,
the temp. outside is 0 to +5C and I'm getting about 24mpg. I don't know
if there is a way to check the knock sensor (Volvo manual doesn't say anything)
but I check the resistance on the old knock sensor 0.3MOhms and the new
one had 4.8MOhms
Running; Bad Knock Sensor. [Symptom:] Higher idle, no power; running
rough at RPM higher than idle. It jerks every few seconds and there is
no pattern to it. It idles and starts fine.
[Diagnosis:] Check the knock sensor, it
senses knock (no kidding) and retards the timing. When this gets faulty,
it will make the engine have very poor power, lousy and jerky acceleration,
but will start and idle fine. It's a small black plastic covered unit bolted
to the block (ten mil. bolt) under the intake man. with a plug attached
to it. One minute to remove. It "listens" to the engine. Only problem is
I can't tell you what to look for when it's faulty. Mine had a cracked
plastic cover and covered in oil. I put a used one in (new they're ~$40)
and it's like I suddenly had a new car.
Running; Cylinder Diagnosis [Query: 1992 740/ B230F] The car
is hard to start and when it does, it idles badly (everything shakes like
it's running on 3 cylinders). Once the engine warms up, it runs smoothly
and the problem disappears. [Response: Chris Mullet] When it's running
rough try to identify a particular cylinder that might be the problem by
pulling spark plug wires one at a time until you find one that has little
negative effect when removed. If you find a bad cylinder, make certain
the intake manifold is properly torqued. While it's running rough, squirt
something like light oil around the intake gasket in the area of the bad
cylinder. If it smooths the idle temporarily, you found the problem. If
not, try swapping injectors between two cylinders and see if the problem
follows it. If not, try swapping injector wires. (You could do this on
the older models, as injector timing wasn't particularly critical, so I
assume you can do it on the newer ones.)
Idle; Faulty TPS or ECT. [Symptom:] In the last 2-3 weeks the engine
idle speed can vary wildly or act normally. Specifically, sometimes upon
first start the RPM's will hold at close to 2000. Then upon warming (3-5
min) the idle speed will swing instantaneously from near die out to about
1600 RPM's. This wild swing occurs only when in park or neutral and never
shuts completely down. The RPM's will stabilize when in gear (auto trans)
and holds at about 1000 RPM's and will hold at about 1600 RPM's when taken
out of gear. The symptom is intermittent. When the engine is acting normal
the idle in park is about 1000 RPM and in gear about 750 RPM. I cleaned
the throttle body (for the third time since owning the car) about a week
before the first time the unusual symptoms occurred. I have disconnected
the AMM wiring during an episode and the engine speed changed but I still
am not sure if I know what to look for there.
[Diagnosis:] What you describe could be
the FI computer trying to limit the rpms at idle so it doesn't over rev.
The control loop is rather crude and is there only to save the engine from
self destroying. Something is telling the computer to rev up and it does
and then cuts off things at 2000 rpm. The most obvious guess would be that
the throttle position sensor (TPS) is mis-adjusted or broken. Make sure
that it "clicks" just before the throttle plate closes. Also, you may want
to check that the FI-unit actually sees this "click" by looking at pin
2 of the FI unit. It should be 0 Volts at idle and battery voltage when
the gas pedal is depressed a bit. You say you cleaned the throttle body
BEFORE the symptoms started appearing, so perhaps there's a chance you
accidentally didn't adjust the TPS position properly?
The other thing to check is the engine
coolant temp sensor (ECT). Look at pin 13 of the FI unit. When the engine
is warm you should see 350mV. See Diagnosing
ECT Failures for more information. In addition, since you've
disconnected the mass airflow sensor (MAF), the computer may have gone
into limp-home mode permanently which will cloud the diagnostics, so you
should really check and reset any error codes stored in the system before
you do anything else. However, unless the Check Engine light has come on
the MAF disconnect didn't trigger any code setting.
Surge and High Idle: Vacuum Leak [Query:] Idle surges at times
between 1200-1600 RPM when car not under load. Idle Control Motor? Other?
[Response: WBain] You have a vacuum leak. Check all lines especially at
the throttle body. Also if it's a turbo, look for a bad intake manifold
Backfiring While Coasting; TPS Mis-adjusted. [Query:] The car
is a 945T and, after fully warmed up 30-45 minutes on the highway), makes
a "pupp pupp pupp" sound when on the highway and backing off the accelerator.
The sound seems to be emanating inside a "tin can" and is not loud at all.
If I had to guess, I would say that unburned fuel/air mixture is getting
into the exhaust system. [Response: Abe Crombie] The injectors should
go off on coast. This is controlled by the fuel control unit being able
to see closed throttle via the throttle switch. If that isn't adjusted
properly or has a faulty internal switch then the injectors may not be
off on coast and you may very well be hearing a weak backfire in exhaust.
Cut-Out While Driving; Electrical Causes.[Symptom:] On occasion, it
will just completely cut out on me going down the road. No sputtering,
just "dead". Usually it starts back up on its own recognizance as I coast
for 50 yards or so. Sometimes it does not, but if I pull off and let it
cool down for 5 minutes, it starts right back up, and does not cut out
again for a couple weeks or so.
Since this car has an EFI system, LH-Jetronic
I suppose, you could be getting an intermittent electrical fault in that
system that'd cause similar symptoms... or I might be totally wrong. First
thing, I'd install a tach. If the tach drops to zero as the engine cuts
out, then it IS the ignition. But I'd clean all the EFI grounds, the underhood
fuse and fuel pump fuses, fuel pump connections, and such before proceeding.
I would check the electrical connections
to the fuel pump. Especially the grounding, maybe it is not so much getting
too hot that causes the engine to quit, but an intermittent ground. Once
you pull over, the broken wire or whatever falls back into place. I once
broke the ground wire to my alternator and that caused the engine to quit,
though it did sputter a little. The wire was broken in the middle and once
I pulled over and the car was still, I had good ground, but once moving,
it would make intermittent contact
Have you checked the fuel pump relay?
Similar symptom to yours and seems to be a fairly common problem among
Bricksters. A bad/weak coil in my experience shows up as consistent poor
performance under high spark load conditions, such as starting and large
throttle openings at low RPMs. I check the coil by removing the coil wire
at the distributor and firmly locating (not holding unless with insulated
pliers) the the wire's tip about 3/8 inch from a block or frame ground
point. The spark should be a "thick" blueish white and easily jump the
3/8" air gap with the engine being cranked by the starter. A weak spark
tends to be "thin" pinkish and sensitive to proximity to ground. The normal
arcing voltage (engine running) is in the 12-15 KV range for my '82 B21F
I have had the similar problem on a 84
GL and found that the ignition pick-up coil in the distributor was faulty.
When the pick-up got warm the resistance in the wire showed an open circuit
and when it cooled off it was normal again. I replaced the pick-up and
problem was solved.
I had such a condition on an '83 245 twice
once it turned out to be fraying wiring harness right between the rear
of the engine and the firewall and once it turned out to be the computer
on the inside of the right front fender.
Cuts Out at Speed: Ignition Power Stage Failure. [Query:] My
1990 740 GL Wagon (186,000 miles) stalls intermittently with the tachometer
immediately dropping to zero. It does it while cruising on the highway
or while idling. I'm able to restart it sometimes with the clutch if I'm
moving along, otherwise with the key after I get to the side of the road.
If it sits for a few minutes that seems to help in restarting. It appears
that all other electrical components are not affected when this happens
(radio works, blower, flashers, etc.). It now happens almost daily, but
started months ago happening 1-2 times a month, then 1-2 times a week,
etc. It's getting worse. My mechanic has replaced some likely components:
FP relay (behind fuse box), radio relay (on the coolant reservoir) and
crank sensor (with the white band - I had replaced this at 90K as well).
I also had a cracked distributor cap that was replaced. Plug wires are
in good shape. [Another similar case:] My symptoms were, the
tach dropping to zero and then the engine would bump start at speed and
at a stop the car would shudder and then die unless the motor is revved
up at a stop to bump start the car. [Response: Lincoln] I would check
your power stage, it amplifies the signal from the computer to the coil.
I had the same thing happen to me two months ago. I think another person
had it happen to them too. I think I paid $90 for the part and $100 for
them to find the problem. The part is located to the right of the battery
on the fender. [Response 2: Fred Guest] In our case it is caused
by a bad connection at a plug connecting the power stage mounted on the
left inner fender. We unplug the connector, clean the spade terminals with
emery cloth and squeeze the female connectors with needlenose pliers if
we have them with us, then put it back together and it is fine for a while.
[Editor’s Note: use electronic connection de-oxidizer instead] I
believe the round things on the fender have something to do with the fuel
injection. Since your tach goes to zero I expect you have a similar corroded
connector problem - but probably in the ignition system rather than in
the fuel injection.
Misfire, Drop in RPM. [Symptom:] The engine idles and runs very smoothly
until the engine is pushed to RPMs exceeding about 3,100 to 3,200 RPM.
Once those RPMs are reached, the engine begins to shimmy, shake and bog
down such that RPMs (if the accelerator pedal is held down) actually drop
down to about 2,200 RPM. Only when the 3,100 RPM "wall" is breached does
the problem manifest itself. The problem occurs regardless of whether the
car is being driven or is sitting in neutral. This is an 89 745, 230F with
an engine from a 93 940. I have swapped or replaced the following items
since engine replacement (and since discovering the problem) without any
noticeable effect occurring re. the 3,100 RPM wall problem: cap and rotor;
spark plugs; spark plug wires; all fuel injectors; fuel pressure regulator;
in-tank fuel pump (new); fuel filter (new); and crankshaft/RPM Hall Effect
sensor(new). I also tested the fuel pressure under static and load conditions.
Under static conditions the fuel pressure on the high side of the fuel
rail is about 45 psi. Under load conditions (i.e., engine running) the
fuel pressure is about 40 psi, and does not decrease when the 3,100 RPM
"wall" is hit (it actually increases a little bit to about 42-43 psi).
The "Check Engine" light is not on. I disconnected the air mass meter when
the car was idling with no noticeable effect on performance occurring.
[Diagnosis 1:] Are you sure the misfiring
is dependent on the RPM and not the throttle position? I had a similar
problem with a Ford, and it turned out to be a worn out throttle position
potentiometer, at certain positions it would hit a bad spot and the ECU
would shut off the injectors. You said that fuel pressure increases when
it starts to miss, that suggests that the injectors shut off or at least
not open as long as they should, so something suddenly starts telling the
ECU that the engine doesn't need as much fuel as it actually does.
[Diagnosis 2:] Perhaps, the fuel pressure
regulator. Had the one on my '87 745TGA go west on me. The darn thing would
hardly hold a hundred with the throttle floored (that one got a laugh from
the techs--they thought I was kidding). But he's right, the switch sometimes
goes bad, too, though I thought I remember you mentioning you had gone
to full throttle and it still was bad. They sometimes use electrical contact
cleaner and a good stiff wire brush on those units (though that happened
more often with the '83 245 than with either 74x).
Ignition Shutoff: FI Relay or Ignition Switch. [Query:] I have
had my Volvo 760GLE for over a year. Currently it has 214K miles. Lately
it has been cutting off (the ignition) while driving. It happens only when
the car is warm, say 10-15 minutes after driving. It first started while
driving on the highways. Without any prior signal it would abruptly shut
of f. (indicated by all the dash lights coming on) once or twice it start
back up on its own a second or two while still cruising and I would drive
as usual. Sometimes I would coast it down to the side of the highway,
try starting a few times, wait, try again, and it would start again. By
the way it shuts off, it seems very unlikely that it is a fuel system problem
(does not sputter or hesitate, just shuts off abruptly) Looks more like
an ignition problem: could it be the ignition module, coil or simply a
poor ground ? [Response 1:] Try removing the fuel injection
relay and replacing. Your symptoms sound very much like a failing FI relay.
Many of them fail through solder fatigue on the back of the board inside,
and you can try to R&R the unit by resoldering where you see cracks
or dull solder. FYI, the FI relay is above the fuse board under the cigarette
lighter. Take off the pop-on cigarette lighter cover, remove the two screws
holding the storage box and pull the box out. You then have full
access to the relays. The rectangular white one in the middle row, driver's
side, is the FI relay. I carry a spare relay around in the
back of my 1990 740 in case of failure on a long trip. They seem
to last around eight to ten years. [Response 2:] In addition to the
fuel relay mentioned the ignition switch can do that to you and it is certainly
a candidate to be failed at 214K miles. I would replace it and see,
if it's not that then it won't be wasted effort as it will fail soon if
it has not already. The switch can somewhat be checked by twisting ignition
key a few degrees either way the next time it does its shutdown number
Driveability Problems with ECU Error Codes; Oxidized Connectors. The
recent steamcleaning problems with a 240 and connector problems with another
240 bring to mind a recent Volvo Tech Service Bulletin from March, 1993
which describes a procedure for cleaning, de-oxidizing and protecting engine
electrical connectors to eliminate driveability problems when unexplained
error codes appear at the ECU. Apparently, they started using protective
grease at the factory in 940 chassis numbers 128400 and 945: 079100; before
this, the connectors are unprotected. No 7xx cars have grease applied at
the factory. It helps to have a Volvo wiring diagram at hand to locate
all of the appropriate connectors, each of which has to be disconnected,
cleaned and greased. You use a cleaning/deoxidizing spray such as "DeoxIt"
from Caig (available at electronic stores) and a low-temperature silicone
dielectric (non-conductive) grease for electrical connectors, available
at auto stores. Don't use "OxGuard", which is conductive. You turn off
the ignition and disconnect, clean and grease the following connectors
leading to the sensors noted:
DON'T do this to the oxygen sensor connector.
The deoxidizing spray is applied to both connector halves, then you squeeze
the grease in and around the connector (but don't fill the protective boot),
then reconnect. After everything is back together, you turn on the ignition
and start the engine. They note that this procedure is supposed to be used
for driveability problems or if oxidation is noticed, but I can imagine
that it would be good practice as a PM technique on higher mileage engines
on older 240/7xx cars.
RPM sensor at back firewall near brake
MAF sensor at the AMM
Power stage at fender
Knock sensor beneath intake manifold
ECT sensor near flame trap
Throttle position switch on throttle body
EGR temperature sensor (California cars)
IAC valve beneath throttle body
Runs but Won't Re-Start; Bad RPM Sensor. I recently posted a starting
problem with a 1991 240, where it would start fine when it was cold and
would start fine hot if restarted immediately after cutting off the engine.
However, if it were left for 10 minutes or more after cutting it off hot,
the starter motor would just turn and turn until it finally started - sometimes
it did not and the battery just ran down. As it turns out, it was the RPM
sensor, which is also a crankshaft position sensor. The heat soak after
turning off the engine made the sensor open circuit most of the time -
during starting, apparently, the computer did not pick up enough crankshaft
position information to start the car. However, since the car ran fine
if it did start, the sensor must have been intermittently putting out a
signal, enough to update the computer in order to correctly operate the
fuel injection and ignition systems after starting. After replacing the
sensor, I have had no further problems with starting. Just thought y'all
might like to know the solution to this problem, in case your brick ever
acts like this. Thanks for all your inputs to my problem.
Stalls When Brakes Applied: Vacuum Leak or FI Relay. [Query:]
I am having a problem stalling under hard braking. I have had it
checked for vacuum leaks and they appear OK. It only happens when
the car is warm and I apply the brakes hard. [Response 1:] I would
look in the direction of the power brake booster. Right after you
brake, you fill up the booster with air, which has to be evacuated by the
engine. If your idle is somewhat shaky, throttle plate is out of
adjustment or you have a lazy idle air motor the idle can drop enough to
stall it. The check valve in hose from intake manifold to booster
has an arrow on it indicating the direction of installation Take
it out of the line and suck on it to see it opens and shuts correctly.
[Response 2:] Check the solder joints on the fuel pump relay. Cracked joints
will sometimes cause a warm relay to drop out when the engine speed drops
and the alternator slows and the system voltage drops.
Stalls at RPM: FI Relay or Hall Sensor. [Query:] Car is 1986
740T with B230 engine and A/T. Occasionally, while accelerating, RPM at
approx. 2500 the engine will start to hesitate, choke, and die. I pull
over to the side of the road where it will restart on first attempt. I
take off and all is well for days and weeks. I thought that once the engine
shuts off, and the car still moving from inertia I could move the
shifter into Neutral, and restart the engine. Starter will Not even
engage. However, with the car stopped, it will start in Neutral. Why is
this ? Fuel pump relay is about a year old. Other than this, car runs and
idles fine. [Response 1: FI relay] It sounds like the fuel pump relay
is cutting out. Behind the ashtray is the fuse box and relay panel. About
the ashtray should be the lighter and a storage bin. Pull the lighter out
and remove the square piece of trim , there are two screws , remove them
and the whole box should come out. The fuel pump relay should be the relay
located directly below the big red round one. It is rectangular and white
the current p/n is 3523608 . The relay should have six terminals
on it. [Response 2: Hall sensor] The easiest way I know of checking
the distributor is to remove it with the plug for the pickup still attached.
Turn the key to the run position, and spin the dist. If the injectors
start clicking the pickup is good. If the pickup is bad , good news bad
news. The good news is there is a replacement pickup p/n 1346792 ( the
black plastic plug is also available it doesn't come with the pickup p/n1346793)
The bad news , the pickup is riveted to the distributor body, and you run
maybe a 50/50 percent chance of breaking the housing. A rebuilt distributor
is available from Volvo it comes with a new cap and rotor, but more importantly
a new shaft seal in the housing that is not available as a spare part (dist
p/n 8111214) Either way get new o-rings for the dist. p/n 969330
Running Problems: General Diagnostic Notes. [Tips from Duane
Hoberg] For the LH system injectors to function properly, three items are
sampled at all times and are used to determine the injector pulse time
based upon the fuel pressure at a specific pressure. Those items are critical
to a normal running engine.
Causes of Running Rich Problems in LH Systems.
High fuel pressure. Check fuel pressure.
If the fuel pressure is out of spec the engine runs rich or lean until
the O2 sensor compensates if the range is not that far out of spec. This
shows up as a cold running and start problem that may go away when hot
with an adverse effect on gas mileage.
Check injectors to make sure they are closing.
Fuel check valve on front of fuel pump. If
stuck part way, it restricts the flow which causes a lean condition. Mechanics
may have compensated by adjusting the AMM. This "cured" the hot running
condition but did nothing for the cold start. As the valve restricts or
opens further, the car runs like crap. If suddenly opens full, the car
will run rich. Associated symptom is it takes a long time to start first
off in the morning. Normal time should be no more than 3 seconds. Anything
longer and there is a problem.
Temp sensor for the ECU is bad or out of specification
or its ground is faulty. From cold to hot the engine needs different amounts
of fuel to run efficiently. The sensor that determines this info needs
to be consistent across its range. If at various points it is out of spec,
open or full closed the engine computer responds appropriately and adjusts
the pulse rate which will lean out or flood the engine. Measurement is
remove sensor, cool it in freezer, attach ohmmeter, place in cold water
on stove and heat to boiling while watching meter, needle movement must
be smooth over entire range. Final test is in engine at full operating
temperature with test point at ECU connector. It must be with 10 to 15
ohms of the chart in the various service manuals.
Faulty AMM. Amount of air is determined
by AMM. If the meter is out of specification the ECU will cause the engine
to run rich or lean. A slight out of spec can be compensated for by adjusting
the mixture control. If adjusting the mixture control does not work AT
ALL engine running conditions, then replace the AMM. Obviously, any air
entering downstream of the meter leans the mixture. An exhaust system that
is not tight or partially plugged changes the amount of air that can move
through the engine and changes the running parameters of the engine. Adjusting
the AMM can compensate until the specs shift too far and then engine performance
and running problems abound. Make sure the charcoal cannister is not faulty
and allowing engine vacuum to suck fuel from the tank to the intake manifold
through it. A definite rich condition.
O2 sensor out of spec or damaged. Once the
sensor gets to operating temp, it provides a compensating siganl to the
ECU which is still running the engine based upon air mass and engine temp
at a specific fuel pressure. The ECU is constantly "looking" for this signal
after a certain engine temp. Once found it uses the signal from the O2
sensor to adjust the pulse rate to control emissions.
The idle control valve only controls
idle. It allows measured air to bypass the throttle plate based upon a
specific fuel pressure and the tach signal.
Stalls; Bad Fuel Pressure Regulator Likely Cause. [Variety of "car
won't restart or is driving along and stalls abruptly" problems.] There
have been a number of these problems recently--the car won't start after
I drive it 20 minutes, the car won't start after I turn it off after driving
a short distance unless I wait several minutes, the car made me wait 20
minutes at the grocery store then started and ran fine...ones like this,
that all sound to me similar to the problem that afflicted me a while back,
which was cured by a new fuel pump relay. My problem was sporadic, and
the symptoms varied, but boiled down to a well-running car simply, now
and then, refusing to start. Given that it now seems clear that the FPR
is fairly often culprit in these cases, and given its low cost, I'm wondering
if maybe trying to replace the fuel pump relay, after checking for loose
wires and that the fuses are content, shouldn't be the first line of attack.
What I mean is that intermittent failure in the FPR is not uncommon once
you get into years and miles, it's something most of us would not have
a hard time replacing, and it seems, from many of the postings, to be not
only the usual suspect, but actually the real culprit. Sure, there are
lots of other things it *could* be, but it seems that this is what it comes
down to more than 2/3 of the time. Starting with the FPR should, on average,
save most people a lot of time I think.
Stalls During Turn; Bad Fuel Pre-Pump Likely Cause. See Fuel
Pre-Pump Problems for more information about pre-pump problems
causing odd stalling situations.
Performance; Rich Mixture Smell: Diagnostics; Faulty FPR
[Query: Jarrod Stenberg] My non-turbo, auto transmission car has been running
real crappy for a while now; I think it was gradual. Sometimes when I start
it it takes forever. This can include some backfiring. When it finally
does start it sometimes spits out a cloud of smoke. It often smells
like gas as well. Seems to be running rich. The oil smelled like
gas. I replaced it since this scares me for good reason. Of course this
improved nothing but my peace of mind. When idling it will race a bit and
cycle back down to near stall (to and fro to and fro but not REAL bad).
Things I have checked and symptoms: Weak spark? Replaced the plugs
and I have new wires. Checked the distributor cap and rotor. All are good.
I have done the easy checks for vacuum leaks: sprayed wd40 around and listened
for the engine to choke on it. I am pretty certain this is not it.
[Diagnostic Notes: Don Foster]
First, it certainly could be a failing
fuel pressure regulator. Pull the small vacuum line and sniff for gas --
possibly the diaphragm has a pinhole and is bypassing fuel directly into
the intake manifold. But even if no gas smell, the regulator could
have increased the fuel pressure creating an always-rich condition -- this
is not uncommon.
Second, my favorite is the connections
at the temperature sensor -- the two-connector sensor under intake
runner #3. (The single connector sensor under runner #2 is for the dashboard
temp gauge.) This sensor uses an NTC (negative temperature
coefficient) thermistor. When the temp is low, the resistance is high,
and when it's hot, the resistance is low. The FI ecu adjusts the injector
pulse duration based partly on this reading to compensate for engine temperature.
So a failed sensor, a bad or broken connector, corroded connections, or
broken wire would create very high resistance and simulate a very cold
engine (like minus 50 degrees). The ecu would adjust fuel delivery accordingly.
Third -- a bad AMM. There's always the
old "limp home" trick. If you find the engine runs well (with the AMM unplugged)
above about 30-35 mph, then you probably have TOO much fuel pressure. If
it were my car in this case, I'd suspect the regulator. But if the
engine can't get above 30 mph, more or less, then it's probably the AMM.
Fourth [Don Willson] Check the ground
wires on the injectors. Remove the manifold bolts and solder the ground
wires to the crimp lugs. Wire brush the connector and around the manifold
bolt and tighten the lugs down securely. A smart mechanic said this is
the first thing he "fixes" on any European car that comes in. When these
ground wires develop a high resistance an injector starts to misfire sending
excess oxygen to the O2 sensor which it thinks is a lean condition and
calls for more gas. Let us know if this works.
[Solution: Jarrod] I pulled the
little vacuum hose off the fuel pressure regulator and it did stink like
gas. Then once started I removed it and it was squirting gas out.
I've found the problem.
Perfomance, Bad Acceleration: Faulty FPR. [Query: Aidan
] I have a problem which I just can't seem to figure out- what would make
my '89 744 GL with just a tad over 105k go from running beautifully smooth
to not-so-good nearly overnight? Basically, the car has been running great
for a couple of months, ever since I had some (relatively) minor work done
on it. Everything has been absolutely wonderful- until yesterday evening.
Literally, all of a sudden the car started giving a great deal of resistance
when I would accelerate- the RPM's would jump back and forth like crazy,
and the car would shudder and jerk when accelerating and idling... (like,
say, at a stop light.) This is all while the car has a full tank of gas.
It began to do this pretty much as soon as I started the car, but continued
even after it had been running for about an hour. The temperature just
dropped a great deal up here in MA, and as it was late at night after getting
home, I didn't really have a chance to take a look at it. I didn't use
the car again until tonight, so it had been sitting cold for about 9 hours.
When I tried it was VERY hard to start, so hard that I feared that for
the first time ever I wouldn't actually be able to get it started. When
it did start, the engine jumped around a great deal between .75 and 2 RPM's
while in Park, and the car continued to shudder a great deal even after
the engine had warmed up and was put in both Drive and in Reverse. The
car seems most unresponsive to acceleration when the engine is running
under 2 RPM's, though trying to accelerate causes the same reaction from
the engine at nearly any speed/RPM. After taking a look under the
hood, I noticed that when the engine can be seen shaking a great deal more
than it should, literally jerking itself back and forth. It often sounds
like the engine is just going to cut out, only to rev up again. (it repeats
this process indefinitely, warm or cold.) Without saying, the car
is very difficult to drive like this, and I fear that one of these times
its either just not going to start, or going to cut out while driving-
both things I'm not looking forward to. Does anyone have any suggestions
about what could cause it do this, or have any ideas about what I should
be looking at/for? Any help/ideas/suggestions would be MUCH
[Final Diagnosis and Fix: Aidan] After
fiddling around and trying a variety of different things, I've come upon
what seems to be the solution- a faulty fuel pressure regulator. After
replacing it first with a test part and finally a new one for $58.87, I
have not experienced any kind of problem in running, idling, or starting.
I came upon finding it pretty much by trial and error- the AMM checked
out and appeared fine, as did the thermostat in the air filter.
Hesitation: Bad Fuel Pressure Regulator. [Symptoms:] occasional stalling
engine at idle; occasional stumble at cruising speed, resulting in 1-2
seconds of deceleration like I'd turned the key off. [Diagnosis:] The bottom
line is the fuel pressure regulator was bad, causing too high pressure;
replace pressure regulator. This whole problem was compounded by the fact
that there isn't a commonly available pressure gauge that fits a Bosch
FI system; even my mechanic has a homemade one. I delayed and delayed getting
the gauge in place, which would've immediately solved the problem when
it first started occurring! Excuses: I was unable to get the fuel rail
to unbolt from the hose to the pump either, so I eventually cut the fuel
line itself, which was a two-piece construction (rubber outside, poly liner
inside). I purchased Sunpro ($35) fuel gauge because it had a hose (with
GM/Ford fittings) that I could remove; and I basically purchased one of
every part at the local plumbing supply shop. Anyway, putting the gauge
in instantly revealed too high pressure. Jumpering the fuel pump was easy
too. And nothing could've been easier than replacing the pressure regulator
-- two screws and it's out. The hardest part was swallowing the $80.00.
Note: Imparts carries nice 1.5" fuel pressure gauges (you mount them directly
in line) #153008 60psi - 1/8" 27NPT connection $21 USSummit carries nice
small aluminum connector for those gauges - hose to hose to 1/8" 27NPT
thread - part # SUM-G1710 $5.99 they have gauges at the same price as well.
Note: I had a hose assembly made at the
local hydraulic shop. the fittings were made by GATES (the hose people)
The sizes are 14mm with a 1.2 mm/pitch and a 37(?) degree JIS flare.
in Oil: Faulty FPR or Injector. [Query:] My brother's 740
with a B230FT has gas in the oil. I lent him my fuel pressure gauge to
check for over pressure, guessing that it was a bad fuel pressure regulator.
Bad news is that his pressure is right on the money, I think he said about
45 psi. Anyway, the only thing we could think of is a leaky injector
that's dribbling when the car's off. The problem with that diagnosis is
that if gas was leaking from an injector, the car would take a few extra
turns to start in the morning since fuel pressure would be down. He says
that it starts right up. I should probably have him leave the pressure
gauge on overnight and check the pressure in the morning. Anyone
have any ideas? [Response: Steve Seekins]
There are only two sources for fuel in
the crankcase. First one is the fuel pressure regulator - however, this
may not affect the working pressure! With the engine running, disconnect
and plug the vacuum line to the intake manifold. If fuel comes out of the
regulator vacuum line, there is a hole in the diaphragm and that is the
source of the fuel. Replace the regulator and change the oil and filter.
This is a fairly common problem.
The second source of fuel in the crankcase
is an over-rich engine. First, if this is your problem, your mileage will
be terrible - on the order of 12 to 16 mpg. If so, it can be EITHER leaking
injector OR a clogged injector, or possibly a bad oxygen sensor. Leaking
injector can be checked by removing the injectors, leaving them connected
to the injector rail. Cycle the fuel pump several times with the starter,
then wipe the injector tips and observe for droplets forming. Also, have
someone crank the engine while you observe the injector spray patterns
(CAUTION _ RAW FUEL WILL BE SPRAYED. BE SURE TO DISCONNECT THE SPARK AND
OBSERVE SAFETY PRECAUTIONS.) Look for spurts, squirts, etc - basically
anything other than a nice even cone-shaped spray. Poorly atomized fuel
may not burn completely and result in some fuel getting to the crankcase.
Also look for a CLOGGED injector. A clogged injector will make one cylinder
run very lean, but the computer which looks at the average will try to
compensate by richening up ALL injectors and net result is engine running
very rich on 3 cylinders and lean on 1 cylinder. Don't forget to check
the cold start injector if installed - some engines have them, others do
not depending on year and type of injection system.
Stalls Repeatedly on Startup: Fuel Pump Check Valve. [Query:]
I have a 1985 740 GLE and every morning when I start my car it will stall
approx. 3 times before leaving the driveway. After this it works fine,
and if I park the car and then come back a while later it starts right
up and is fine. It just seems to have a problem when it’s been sitting
a while, i.e. overnight and getting to/leaving for work. It has new Bosch
platinum plugs as well as new ignition wires. And this problem occurs
regardless of the temperature both summer and winter in upstate NY.
[Response: Steve Seekins] This sounds like a classic fuel pump check valve
problem. The check valve is there to prevent the fuel from draining back
to the tank when shut off. In the AM, try ticking the starter just enough
to make the fuel pump run for a second or so - but not enough to start
the car. Do this 3 times, then start the car. If it starts and runs normally
without stalling, replace the check valve located back at the fuel pump.
Other things to check - make sure that throttle body is clean, check injector
seals, wiring harness, etc. Clean, replace, repair as needed. Consider
running a can of BG44K through the fuel system to clean it -particularly
if this car has not had regular maintenance of the fuel system.
Stalls, Lights Die: Electrical Ground Fault. [Query:] Does anyone
know if the bulb out sensor box can cause a dim headlight intermittantly.
Sometimes if at a stop light or parked, if I turn on the brights the engine
will die. Also sometimes the rt. turn sign and bright indicator on the
dash appear to be dim when they shouldn't be lit at all. [Response:]
There is a ground bar on each inner front fender. Make sure all of these
wire connectors are clean and tight. I forget what else grounds here but
a dirty/loose ground at this grounding point will cause the engine to die
and also dim headlights. Check both ground points for the inner fender
area for being loose and/or dirty.
Cold Idle Problems -Bad ECT or O2 Sensor and Wiring Harness Notes (BB)The
block temperature sensor (ECT engine coolant temperature sensor under the
intake manifold) plays a big part in cold running decisions and could pre-maturely
allow the O2 sensor signal being used before the engine is warm. Because
of this a cold idle problem rarely involves a faulty O2 sensor, but may
involve the temp sensor and more specifically the wiring at the temp sensor.
My experience is that this usually leads to quite rough running at all
times, but fast idle and poor acceleration are known symptoms. With the
ignition on and all wiring in place, you should see voltage at the temp
sensor terminal connected to the blue (or orange) wire that goes back to
the ECU. See Diagnosing
ECT Failures for more information. No volts means broken
wiring or bad ECU. A bad ECU may actually just be a bad ground at the ECU,
so be warned. A cold engine should read 2-4 volts, a fully warmed engine
0-2 volts. If you see the full supply voltage of 4-5 volts (the reading
you should get from the ECU when the connector is pulled off the temp sensor)
then the temp sensor or its ground wire are faulty.
The temp sensor wiring goes into the wiring
harness and along the firewall before joining with the O2 sensor wiring
and going through the firewall to the ECU behind the right side kick panel.
If you have a general wiring harness deterioration problem then a fair
bit of digging and careful tracing may be required to isolate the problem.
You can splice in your own repairs, but for extensive problems a total
wiring harness replacement may be needed.
Automotive wiring from your local retailer
is often not up to the job of being near a hot engine. If possible, make
your splices using heat resistant wiring (like oven/stove wiring from an
appliance parts/service shop). Also use heat resistant connectors (nickel
or copper) rather than auto grade (aluminum), you should be able to get
them at the same place. Heat shrink tubing can be used to insulate the
connectors and can also be used to insulate short runs of bare wire. Better
quality shrink tubing is available from electronics/electrical or appliance
BTW Cheap PVC clad wire, electrical tape,
shrink tubing, dielectric grease, etc. should not be used at the O2 sensor
lest it melt and burn from all the exhaust manifold heat. If in doubt,
test a scrap for heat resistance.
So, with wiring harness problems in mind,
resolve to keep your engine compartment a little cleaner so that engine
oil and road grunge buildup on the wiring doesn't hasten deterioration
of the insulation. If your engine is weeping oil onto the head then get
the valve cover gasket and/or camshaft seals attended to. You don't have
to become a fanatic and start waxing the firewall, just use detergent and
an old wash mitt and maybe the occasional spray-can of engine degreaser.
When rinsing off, avoid drowning your distributor and ignition control
unit. Everything else is pretty much waterproof (actually the ICU is also
normally waterproof, but why take chances).
Won't Start: Neutral A/T Safety Switch at Fault. [Query:]
Starter will not operate when ignition turned to "start". [Response:]
I've had a starting problem with my 89 744GL, and discovered that the 'neutral
safety switch' was the culprit. It's a device in the base of the (automatic)
shifter that will only allow the car to start in neutral or park. Mine
had worn to where the contact wasn't always made in neutral or park either,
so I bought a new one at the dealer ($48) and it was simple enough to install.
Won't Start; Plugged Catalytic Converter. I hate to post another "my
car won't start" question, but "my car won't start!" This is an '87 240
B230F, 196k miles, LH-J 2.2, Chrysler Ignition with Bosch Distributor.
The car cranks over properly. Fuel pumps both verified running. Injectors
generate "clicking" noise indicating operation. Measured resistance of
all 4 injectors - all correct. Sparks happen at spark plugs. Verified distributor
points at #1 when both #1 cam lobes point up. Spark plugs fire at correct
time (as close as I could estimate with timing light.) Tested all I/O to
ECU - all points had correct continuity/resistance/voltage per Bentley
testing procedure. Tested Air Mass Meter - proper voltage & resistance
on all points; no change when unplugged. Tested fuel pump delivery volume
- ok. Checked resistance of coil - within spec. Suspected gas; drained
tank and fuel lines, and added 3.5 gallons of fresh fuel. Installed new
Cap/Rotor/Plugs/Wires. Verified proper operation of throttle switch. Compression
good at 190/190/190/180. Two instances of flaky wiring noted, on oil pressure
sensor and temperature sensor; verified both not shorting or grounding.
All other wires look clean and almost new. Still no go. At this point,
the car *almost* runs when you crank it. It sounds like it would if you
just cranked it for long enough, but it never does. Occasionally, after
standing a while, it will run badly for 10-15 seconds before dying. Spark
plugs are wet after cranking. I have run out of things to check - I'm stumped.
[Diagnosis:] Have you checked for a plugged exhaust system? [Result:] I
pulled the plug from the test port just ahead of the cat, and it started
and ran, with a loud hissing coming from the port. I disconnected the catalytic
converter at the inlet, and found that the cat had broken up inside. A
large piece had bounced forwards and become firmly lodged in the inlet,
blocking almost all flow. I removed the piece, and the engine started up
promptly and ran very well, albeit loudly.
has Poor Acceleration; Diagnostics. [Query:] Symptoms: When I
give it gas the turbo boost goes into the yellow, however, acceleration
is sluggish. The car fights me all through acceleration. It
starts fine and has a new turbo. My mechanics diagnostic computer
finds no problems.
[Response 1: Paul Grimshaw] Assuming
that the car has been tuned-up regularly (wires, plugs, filters (including
gasoline filter)) and is otherwise in good working order, I would check
1. Check for vacuum leaks. This
can be done with a piece of rubber hose attached to a propane torch.
Open the valve, do not ignite the torch, and move the hose around the intake
manifold. Whenever the idle increases slightly, you have found a
2. Check the fuel pressure regulator against
specification (it is around 43 psi, I believe). If the diaphragm
is shot, your engine is not receiving enough fuel;
3. Examine all of the underhood wiring
carefully. Turbos can be very hard on the wiring and you may have
a number of hidden electrical problems. I've seen 10 wire bundles
almost devoid of insulation due to the effects of oil and heat. Replace
any faulty or damaged harnesses. You could try to solder in repair
pieces, but it is always better to replace the harness!
4. Try replacing the coil. Yes,
coils can be checked, but an intermittent fault in an ignition coil is
very difficult to re-create.
5. [Response 2: J. Dally]
I would also suggest checking for a plugged catalytic converter.
your turbo died, it probably blew oil
past its seals. That oil then clogged up your cat.
Misfires; FI Resistor Pack Defective. [Symptom:] My 86 Volvo
760 turbo is running very rough.. when I tested the cylinders the
#2 cylinder wasn't firing. After many hours of frustration I found a relay
which is located next to the battery. This relay is a little box with
four cylinders in it The cylinders are about 3 inches long with a radius
of about 3/8 of an inch. five wires go into the box One attached to each
cylinder and one going down the center. I presume the lone one is the constant
power. On this box I saw one wire was dislocated. I re-attached this wire
but the cylinder still wasn't firing. I then took of another wire and the
#4 cylinder stopped working. I then attached the working electrode (off
the box) to the #3 cylinder but it still wouldn't fire. I also though have
a periodic problem. Sometimes when I am driving normally my car will start
to misfire. If I floor it will eventually catch (3-4)seconds later. If
I keep it floored it does go.. but when I release the gas to 1/2 it starts
to putter out. Could this be the same problem? What is the name of this
part? [Response:] What you are describing is the resistor pack for the
fuel injectors. Only the turbo's have them. The Volvo p/n is 3531339 and
Volvo lists it for $59.39 dollars US. There is one resistor per injector.
We have seen the batteries corrode the connector over a period of time,
from the lack of battery maintenance.
Start Problems: Faulty Hall Sensor. [Note from Steve Seekins:] Note
that if your car is a turbo, you do not have the crank position sensor,
but you do have a hall effect sensor in the distributor that can
also be the problem (cars with the crank position sensor do not have
the hall sensor in the distributor).
Start Problem: Power Stage Overheats. [Note from Boris]
I had a hot start problem on my car. It drove me bananas. I would pop the
hood just slightly after each frequent stop, and this reduced the frequency
of the problem drastically. Why do you ask? Volvos, especially the
turbo models generate enormous amount of heat once parked after a drive.
This heat has no place to escape. It just sits under hood for a LONG time
and can damage various components. I believed whatever part was malfunctioning
must be under hood and not on relay board. Replaced Hall sensor needlessly.
Problem was in the Power stage. Took it off, cleaned the contacts very
well, and coated with dielectric grease. Next, I coated the heat sink side
of it with thermal compound which to my surprise was not there before.
This helps keep it cooler. For extra protection I taped a styrofoam cup
over it (Yes I too can be frugal…) Problem gone. I was stuck the
other day going 11 miles in 3 hours, and the car ran fine. It was so hot
under the hood, I could have baked a good lunch under there.
Control Problems: High HC, CO or NOx.
Below are some generic diagnosis notes (non-Volvo-specific) to help you
begin pinpointing why you failed the smog/emissions/MoT etc. tests.
[Excerpted from "Exhaust Emissions Diagnosis:
The Precursor to Finding Engine Performance Problems", Larry Carley, ImportCar,
June 2000 at http://www.underhoodservice.com/magnav.htm]
GENERAL DIAGNOSIS. Elevated hydrocarbon
(HC) emissions usually indicate ignition misfire due to fouled spark plugs
or a bad plug. But high HC emissions can also be caused by burned exhaust
valves (check compression), lean misfire (check for vacuum leaks, low fuel
pressure or dirty injectors), or rich fuel conditions (excessive fuel pressure,
leaky injectors or a dead O2 sensor).
High carbon monoxide (CO) emissions are
a telltale sign of a rich fuel mixture. On newer vehicles with feedback
fuel controls and fuel injection, leaky injectors, excessive fuel pressure
and sluggish or contaminated O2 sensors are all possibilities to investigate.
Harder to diagnose are elevated oxides
of nitrogen (NOX) emissions. Causes here may include a defective EGR valve,
EGR vacuum solenoid or motor, plugged EGR ports in the manifold, over-advanced
ignition timing or engine overheating.
IDLE EMISSIONS. A vehicle that has
sharply elevated HC or CO emissions at idle will usually have a noticeable
misfire and/or rough idle. The most likely causes here would be:
• Fouled spark plug(s);
• Shorted spark plug wire(s) or defective
• Vacuum leak;
• EGR valve stuck open;
• Burned exhaust valve;
An extremely rich fuel condition can also
cause elevated HC and CO at idle, while an extremely lean condition will
only cause HC to rise abnormally. A leaky EGR valve can act like a vacuum
leak and cause a lean misfire at idle. HC and CO will be somewhat
higher as a cold engine warms up because the fuel system may still be running
in open loop. Until the engine reaches a predetermined temperature and/or
the oxygen sensor gets hot enough to produce a good signal, the ECU will
supply a relatively rich mixture while the system is in open loop. A faulty
thermostat that is stuck open or a defective coolant sensor may prevent
the system from going into closed loop.
NOX emissions are always lowest during
idle and decel because that’s when engine load and combustion temperatures
ACCELERATION EMISSIONS. During acceleration,
the engine momentarily drops out of closed loop and receives a richer fuel
mixture for more power. During this time (depending on the system), the
Airflow Sensor and the TPS sensor play critical roles in controlling the
Most fuel-injected engines have either
a throttle position sensor or switch that indicates when the engine is
at idle. When this device indicates that the engine is no longer at idle,
the on time of the injectors is increased to temporarily richen the fuel
mixture. The same thing happens any time the engine comes under load and
manifold vacuum drops. The AMM sensor tells the computer the engine is
under load, and the computer responds by adding more fuel.
It is normal to see some spikes in CO
during acceleration, but unusually high CO readings indicates that the
fuel mixture is too rich. Possible causes might include:
• Flooded charcoal canister or a leaky
• Defective mass airflow (MAF) sensor,
• Defective throttle position sensor.
If the feedback fuel control system is
working properly and there are no apparent sensor or purge valve problems,
the catalytic converter may be contaminated or not functioning.
Elevated HC readings during acceleration
indicate ignition misfire under load. The causes could be:
• Defective knock sensor;
• Weak ignition coil(s);
• Excessive resistance in spark plug wires;
• Arcing inside the distributor cap;
• Worn, fouled or incorrectly gapped spark
• Over-advanced ignition timing; or
• Lean air/fuel mixture.
NOX readings will rise sharply during acceleration
and will peak a few seconds after the
cruising speed is reached. If the EGR
system fails to recirculate exhaust back into the intake manifold, combustion
temperatures will rise causing an increase in NOX. The higher temperatures
may also cause some detonation (spark knock) to occur, which may be audible
when the engine is under load. Causes of elevated NOX emissions during
• Defective EGR valve;
• Leaky EGR valve plumbing or control
• Carbon deposits in EGR manifold passageways;
• Carbon buildup on pistons and in combustion
• Over-advanced ignition timing;
• Defective knock sensor;
• Engine overheating (check thermostat,
fan, coolant level);
• Exhaust restrictions.
CRUISE EMISSIONS. At cruise, the
engine is lightly loaded and running at high rpm. Under these conditions,
HC and CO should be low if the oxygen sensor and feed back control system
are working properly, and the catalytic converter is in good condition.
High CO readings during cruise indicate
a rich fuel condition. Causes here may include:
• Defective O2 sensor;
• Exhaust leaks upstream of the O2 sensor
(check manifold gaskets and air plumbing
• Defective AMM sensor;
High HC during cruise would indicate a
steady misfire or loss of compression (leaky exhaust valve).
DECEL EMISSIONS. When decelerating,
the engine will typically either lean out the fuel mixture or shut the
fuel off completely (some fuel-injected engines). The computer typically
uses inputs from the Vehicle Speed Sensor, TPS, Airflow sensor, and engine
rpm to determine when this occurs. When the throttle closes and manifold
vacuum shoots up, the computer cuts back on the fuel. Normally, HC, CO
and NOX emissions drop during deceleration because the engine is no longer
under load and is receiving little or no fuel.
If CO emissions remain high during deceleration,
the engine is receiving too much fuel.
Causes may include:
• Leaky fuel injectors; and
• Faulty VSS, TPS, or airflow sensor.
FAQ for 7xx/9xx/90 Cars
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