1988 Audi 90 idle oscillation

Greetings all, I am investigating the following:

Vehicle: 1988 Audi 90 4 cylinder. New Plugs, Plug Wire set, Distributor cap. No (apparent) vacuum leaks, good gas mileage, Clean ISV, most connectors disassembled, cleaned.

When car is warmed up, idle oscillates between 800 and 1200 RPMs about once a second. When this occurs, the control voltage to the ISV drops from about 4.0 (average DC reading, actual signal is AC) to zero. This allows the ISV to mechanically oscillate from closed to open resulting in the idle oscillation. I have thoroughly cleaned the ISV, and it works perfectly until after warm up. This suggests a bad sensor signal/connection telling the ECU to stop controlling idle ??? The temperature dependence has less than 100% correlation, but close.

Any insights, similar experience data would be helpful and most appreciated.


Thanks,