[V8] Re-birth of my '90 200 tq

[Kent McLean]

Brian K. Ullrich wrote:
> I do have a couple of issues with the driveability of the car. Two areas
> concern me, and they are likely related. 
> 
> First, the idle is a little rough once the car is warm, and it is prone to
> stalling at idle then.

My guesses, and easy fixes, are a bad O2 sensor that doesn't do anything until 
the engine is warm, when it's failure is noticed, or a bad/dirty ISV (idle 
stabilizer valve). A dirty ISV can be cleaned: 
<http://www.audifans.com/kb/ISV_cleaning>

> Second, there is a buck at 1.7 bar. Under that, she boosts well and pulls
> hard, but once I hit 1.7, she bucks until the boost drops below 1.7. I can
> reproduce it at will. This one stumps me.

My thoughts (and they're just that): the turbos have a knock sensor that may be 
activating under extreme boost, causing the ignition to cut out. That might make 
it buck. The turbos also have something (I forget what; helpful, huh?) that cuts 
  boost to prevent over boosting. That might make it buck. And last, old/bad 
spark plugs or ignition components might underperform at high boost, making it buck.

I cc'd the main list, as they might have more ideas. Good luck with it.

-- 
Kent McLean
1990 V8 w/5-speed and other mods
1991 200 TQA #3, with mods
1999 A4 Avant, V6 Tiptronic
gone: '91 200 TQA x2, '94 100 S Avant, '89 200 TQ "Bad Puppy"