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mysterious 5kcstq problem

My Audi Friends,

Please help with this mysterious 5000 TQ problem.


Car starts fine, idles fine, and runs out fine until you come into boost.  
On boost the car begins to buck and breakup.  The behaviour is temperature 
sensitive, in that, if ambient is below 40 deg F, no breakup until say 1.2 
bar.  If ambient is above say 65 deg, occurs around 1.0 bar, and as car 
heats up to full operating temperature the threshold drops to as low as 0.7 


1986 5kcstq 130k miles.  Car has always been a strong 1.4 bar car, and was 
until a service early this summer.  Mechanic is Audi and Porsche certified, 
and is local dealer's shop foreman (not a hack).  Early this summer car was 
serviced for timing belt and basic CIS setup.  I was told that car had been 
set rich and was leaned out.  At this point problem began.  Timing belt 
setting was triple checked and is OK (have checked myself as well).  We 
reset mixture to rich but problem persisted. Mechanic and associates (other 
dealer wrenches) felt it was ignition related.  I've always thought fuel 
mix.  Replaced the following:

cap & rotor
plug wire loom
fuel filter
air filter

Nothing.  Still breaks up.  Checked all vacuum lines, OK.  Pressure tested 
intake system w/ no critical leaks.  Injector seals were thought to be a 
little leaky, and so were replaced and resealed. WOT switch tested and 
declared OK (I've double checked this, as well as followed Scott Mockry's 
test routine, is OK).  Swapped the following parts from wife's 86 5kcstqw:

coil unit
frequency valve

No change.  Checked spark to all cylinders with road test equipment.  All 
fine but with unusual spark maps (acting lean).  Checked fuel pressure.  
Found delivery pressure was fine, but control pressure from differential 
regulator was bad (no control of pressure) and so replaced.

We all thought this was it.  High pressure from regulator back to CIS would 
press down on plate and lean out mix (more noticeable under boost and when 
warmer).  Reset base CIS settings, and test drove.

Nothing. At 0.8 bar buck and breakup.

At this point we are all stumped.  Next step would seem to be a full 
teardown of the CIS system.

Seeking BTDT's and suggestions.  Please reply.


R.H. Reed
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