I just bought a 1990 Coupe Quattro 2.3 20V. Beautiful car, but it has a misfire which appears above 3500 rpm on light throttle openings (i.e. steady 80mph cruise).
It feels like one cylinder is cutting in and out. Full throttle usually clears
it for a short time, then it comes back. If I turn the ignition off and back on while driving it goes away, until I get up to 3500 rpm again. After driving for any length of time on the highway, if I stop, then there is a severe lack of power when pulling away - with full throttle this clears around 4000rpm and the engine then pulls cleanly. Idle is slightly rough but not seriously. Car has done 184,000 miles.
So far I have replaced plugs, air and fuel filter (they needed doing anyway).
I also did a compression test - 190 psi even across all five cylinders.
Questions: does anyone have any idea what this fault is? I am wondering if it
might be a faulty knock sensor. Also, if there is a fault which is stored in
the engine management system's memory, is there a light that comes on to warn you that there is a fault?
Today I retrieved the fault codes and I get 2142 and 2144 which are knock sensor faults. I checked the wires and harness connections for the sensors, they looked fine. I re-torqued the knock sensors to 7 ft.lbs. Should I just go ahead and replace the knock sensors? #1 sensor first naturally. Is there anything that might save me from having to pay for expensive knock sensors?
Any help is greatly appreciated.

 

I got it fixed. The #2 knock sensor was bad.

 

Glad you found it....

Sorry I didnt see this sooner. Thanks for the update.