[V8] Help Diagnosing V8 Problem...

[Buchholz, Steven]

... 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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