1988 Audi 90 idle oscillation

whimorris · #1 07-05-2005, 02:40 PM

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, Ian.

9foraudi · #2 08-03-2005, 12:56 AM

Hi Ian,

I have had similar problems with idle oscillating on my 89 200. Recently I replaced the fuel filter and this seems to have eliminated the idle oscillation. The idle is now right where it should be and fluctuates very little. Plus the throttle response has improved and acceleration is noticable better. I think the new filter removed constriction on the fuel flow and allowed a constant presure level which inturn leveled out the idle.

Rob

rhibdrivr · #3 10-04-2005, 12:11 AM

Sounds like you have a bad idle switch. Again, something I have just dealt with. the plunger fell out of mine. There is an idle switch underneath the throttle body and a full throttle switch on top. idle switch needs to be closed when throttle is closed and tull throttle switch closed when full throttle. If the idle switch is not closed or the full throttle is closed the ISV will not even be operating. There is a 3 wire connector right next to the full throttle switch. You should be able to hear the idle switch click when you open/close the throttle by hand (engine off). Unplug and check continuity between 1&2 and 3&2...the brown with yellow stripe is the ground, blue with white is idle and white with violet for the full throttle. Obviously when checking the full throttle you need to close that switch. Throttle closed, idle switch should be closed.