10 I have a very rough idle/stalling/hesitation/bucking problem when the car is cold (or warm in some cases). What can I do to fix it?
Description
This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
10 I have a very rough idle/stalling/hesitation/bucking problem when the car is cold (or warm in some cases). What can I do to fix it?
A: This is an old problem that may have numerous causes. Mostly A2
Golfs/GTIs/Jettas/GLIs (8v & 16V) are affected. First make sure everything
"obvious" is ok: vacuum (hoses, pipes, intake), electrical connections,
tuned up right, spark plugs, distributor, good tank of gas, etc.
These are by far the most likely causes of this annoying problem.
[jan, 19930902, overheard in the VW shop]
VW is going to embark in some form of campaign, not sure whether it is an
actual NHTSA mandated one, to replace all the ECU's with one using gold
plated connectors, replace and reroute several vacuum hoses, and replace
the throttle body (the shaft has a radial play causing a vacuum leak in
some cases). I believe that recent Passats, A2 Jettas and GTIs are included
in this.
Depending on the model, do the following:
- Change to a different brand/type of gas (4-5 tanks) VW had a bulletin out
on this. They recommend Shell, Chevron & AMOCO(?)
- Use a good Fuel Injector Cleaner every 4000 miles. See Chemicals
for recommendations.
- Use fuel dryer (using ISOPROPANOL NOT Methanol)
- Check (idle) throttle switch
- Clean sensor plate or airmass sensor & throttle body orifices
- Change the fuel filter
- Clean idle stabilizer VALVE with brake or carb cleaner (VW&P) (Note:
Earlier cars had a Digital Idle Stabilizer circuitry (DIS) which is
something different, and *rarely* fails).
For RD (8V) series idle stablilizer problems (& others probably):
Symptom: Idles extremely rough when cold, improves after warmup. Doesn't
compensate for A/C compressor load when on.
Diagnosis: Disconnect the connector from the valve, measure resistance
from the center pin to each side, both should be about 12.5 ohms.
If either one reads high or open, replace it. (will be about $200 at your
Bosch dealer, owww).
Reason: The internals of the valve form a bidirectional DC servomotor,
the windings of which are connected through a commutator, which has a
nasty habit of arcing until the connection is gone.
- Check *all* vacuum hoses (inc. those going to the brake booster & the
brake booster itself)
- Check *all* air pipe connections (esp. between the throttle body & air
cleaner housing). Look around hose clamps, crimped ends, where there may
be relative motion and cause a tear.
- Check for vacuum leaks anywhere else, e.g., around the intake manifold.
- Check *all* electrical connections (see also later on bad grounds)
Be aware of the old "leak in the windshield molding or firewall
gommets that drips and shorts out the fuse box" problem.
Many A1's suffer that problem, especially those with badly installed
replacement windshield (most places, BTW).
- Check inside the distributor (carbon build up will cause misfires/bad idle)
- Check warm-up regulator/thermo switches
- Check proper working of the Oxygen [Lambda] Sensor (see Bentley)
Hint: Disconnect the O2 sensor, if the car runs better suspect that it
may be bad.
Replace if suspect. Some O2 sensors will last longer than their
expected 30/60k miles lifetime, others fail *much* sooner.
Resetting the warning light for maintenance is NOT a good substitute.
- Clean contacts of the ECU, and all engine management related components
with an aerosol contact cleaner (note: Newer VWs use gold plated
contacts, so this definite a problem area in older cars!)
- Check ground of ECU. There is a bulletin out on this. Usually this
failure is acompanied with black smoke billowing from your exhaust.
- Check fuel pump relay, it may have an intermittent failure. HARD
to diagnose, until it cuts out entirely. If it does die,
jump the two large terminals on the relay block to operate the pump
so you won't be stranded. [frank.sikernitsky@mail.trincoll.edu]
- Check the working of the injectors (incl 5th one).
- Check the injector O-rings (older cars)
- Clean the tank screen at the bottom of the tank or on the transfer fuel
pump (& also clean the tank if you find junk)
From WENDTM@FIRNVX.FIRN.EDU (Mark): When I pulled the hose off the intake
side of the fuel pump it only dribbled slowly from the tank! No Gusher!
That was a real clue that the tank was faulty, and not the pump. :)
- Clean the screen *inside* the fuel pump.
- Check whether the filtering banjo bolt (has a screen) near the fuel
distributor has been removed at the first service (mostly A2 cars with FI
in the US, don't know about Europe). It's replaced with a bolt w/o a
screen (Part nos: Screw = N 0210715 Washers = N 0138128, for *most* A2
cars). If it is left, it may clog or restrict flow.
- Check the health of the fuel pump(s) (measure the amount of current it is
drawing). Note, many A2s have two pumps!
- Improperly grounded throttle position potentiometer
(90 Golf: scottz@pangea.Stanford.EDU), as well as VR6s
- VR6 models: Connector to the airmass sensor gone bad.
- VR6 models: Engine temperature sensor gone bad.
For a 1994 Corrado VR6 the temperature sensor in question was
FP NUMBER: 025-906-041-A.
- Check the CONTROL PRESSURE REGULATOR. Apparently the heating element
wears out, and it won't give correct pressure until it warms up
- Faulty oil pump relief valve. Pumps up the hydraulic lifters too much
limiting compression. Apparently mentioned in EuroCar.
- If nothing helped, you may need a new ECU! (The 91?,92? Jettas went
through 5 different ECUs, according to my mechanic).
- Catalytic converter clogged and breaking up. However, if that happens
it will rattle like a coffee can with coins in it.
- If the car bucks/loses power around 3000-4000 rpm when accelerating,
check the full throttle switch. seibed@lamar.ColoState.EDU (Edward Seibert)
- >>>> MORE????
- For Vanagons, see VANAGON_Stall in the archives.
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