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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
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:
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.
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