[V8] Help Diagnosing V8 Problem...

[Alan Kramer]

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

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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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