Sorry if this has been asked before, but I searched and didn't find a similar problem. I have a 1989 200TQ (MC 10v, with a wastegate spring good for 1.7 bar)and this problem has been occuring for a little while now. When the car is cold, it runs smoothly. After it has been running for a few minutes, (not even up to operating temp) it has a cylinder misfire from just off idle to roughly 1700 RPM. I don't have a scope, so I can't tell which cylinder it is. (Though the flashes from a timing light on each wiredon'tlook erratic.)I cannot find a vacuum leak anywhere. I have replaced the injectors with a known good set (with new seals), and the throttle body to intercooler hose was cracked so I replaced that as well. I have alsochanged the plugs, wires, cap and rotor; but the same issue remains. The tune up was needed anyway, but the spark plugs still look normal upon removal. Another thing that I've noticed is it acts as if it's turning back the ignition timing once the boost hits 1.2bar, even at part throttle. It's very noticible, almost like when you turn the a/c on in an underpowered car in the middle of accelerating. Funny thing is, every now and again, if you let off a little and then floor the accelerator it pulls cleanly; but the low RPM miss is always present. I also tried going back to the stock wastegate spring, andfound no codes in the ECM then either. Am I missing something? Thanks in advance, any help is appreciated; as I've onlylearned enough about this car in the past 1 1/2 years of ownership to be dangerous.

 

Check fuel pressure. That sounds like what mine did when the control pressure regulator went bad, it basically makes the car go lean when the control pressure is too high and the turbo is doing everything it can to suck air past the metering plate.

 

Have you replaced any temp sensors? I would check them with an ohm meter when cold and again when warm. These old sensors do fail or become out of spec. Look at http://www.sjmautotechnik.com/troubl.../10vcoolt.html for specs.

Also try posting your question here, it gets alot more traffic than this site for these older Audis.
http://forums.audiworld.com/v8/

Good luck, Tom