[V8] Help Diagnosing V8 Fuel Problem...

Tue Feb 3 14:41:04 EST 2004

I appear to have a similar issue with my V8.  It appears to be fuel delivery
issue.  I have a stumble or miss, very noticeable at highway speeds when
holding a constant speed.  You can hear the motor go flat for a few seconds,
then clear, then flat, it seems like fuel delivery is being constrained in
some way.

I have new wires, plugs, caps and rotors, pulled plugs and all appear even
in appearance, a little lighter brown than what I would consider normal,
however I'm not certain I know what they should look like.  I'm used to a
nice light chocolate brown color on a properly running motor. 

Recently replaced O2 sensor within last 15K.  Checked the wire splices on
the O2 sensor last night, great shape, I used those screw together
connectors with the rubber boots around them, they have worked great.  Good
test recently as we have received a couple inches of freezing rain, sleet,
slush and snow combination the last couple days, excellent Quattro weather!
Also put new fuel filter on, and lubed center ujoint and emergency brake
cables while under the car. :-) 

1. I understand we can sequence through the injectors and listen to them
buzz-Will an injector buzz and still not deliver fuel, I assume a clogged
injector will still appear to function with this test.
2. How can I test for injector performance?  Clogged, damaged tip, etc?  
3. Can you take the fuel rails and injectors, as an assembly, to a shop to
have them run through a cleaning process?  Or does this require the rest of
the car for some reason?
4. Any other ideas?

Thanks!

Bob
90 V8Q  134K Pearl Gray Sports

-----Original Message-----
From: Buchholz, Steven [mailto:Steven.Buchholz at kla-tencor.com] 
Sent: Tuesday, February 03, 2004 12:22 PM
To: 'v8 at audifans.com'
Subject: RE: [V8] Help Diagnosing V8 Problem...

... if you're sure that it is going lean then it isn't spark ... check to
see that the injectors are getting operated.  The ECU fires injectors in
pairs ... and there's an output test you can run to sequence through them
... might help you get to the bottom of the problem ... 

I suspect that if you lost fuel to one or more cylinders that the ECU would
not be able to recognize this as a fault ... it would probably think its
doing a better than average job ... ;-)

Steve B
San Jose, Kaleefohnia (USA)
> 
> Need to ask some advice on diagnosing this problem in my
> friends '90 V8.  
> Here is the data:
> 
> * Engine starts fine, runs fine when cold. (Coolant Temp < 50C)
> 
> * At temperature it seems 4 of the cylinders drop out.   Car 
> continues to
> run, but cannot accelerate or go up hills.
> 
> * VAG com pulls NO CODES besides O2 sensor out of range, intermittent.
> 
> * The cylinders only drops out only at the onset of a
> moderate/high load 
> condition (moderate throttle change, starting off, uphill sections of 
> road....)
> 
> * Letting off the throttle and letting the car idle for a few seconds
> "corrects" the problem.
> 
> * Gentle application of throttle does not produce problem
> until over 4,500 
> RPM.
> 
> * Monitoring the O2 sensor on Vagcom shows over lean
> condition as a result 
> of the cylinders dropping out, which causes the code to appear.
> 
> 
> Diagnosis #0:  Not a fuel problem because no codes are present.
>   Engine continues to run, albeit poorly when problem occurs,
> so fuel pump, 
> fuel pump relay, etc... are ruled out.  Would expect fault 
> code if pair or 
> more of injectors not firing.
> 
> 
> Diagnosis #1:  Ignition failure on one distributor.
>    Replaced both coils with known working ones from my parts
> car.  Problem 
> still persists.
> 
> Not expecting this to do anything, we swapped caps/wires with
> the ones from 
> the parts car and installed new spark plugs.  Our 
> expectations were met.
> 
> 
> Diagnosis #2:  Intermittent short/open on G or G/Bk wires
> from ECU to coils.
>   Continuity tests pass, even while wiggling wires.
> 
> 
> Diagnosis #3:  Faulty ECU.
>    Before we go digging the parts car out of the snow and ice
> some more, is 
> there anything we've missed?
> 
> Thanks,
> Alan
> 
> -----------------------------------
> Alan Kramer
> http://www.geocities.com/audikramer
> 
> '83 CGT 20vt
> '86 4kcsq
> '90 V8q
> '83 UrQ #519 (recipient)
> '91 V8q (engine donor)
> 
> -----------------------------------
> 
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