There are a lot of inputs to the idle speed controller and it could be one of many things but the most common are the failure of the "air slide valve" which pumps air into the intake manifold on idle (which is necessary because it uses a mechanical air flow meter that shuts off the intake manifold at idle). When the Air Slide Vlave fails it burns out the control module and they usually get replaced in pairs. The old system had a manual screw type valve. To fix yours, the best way is to replace the two parts described above. How many miles on your car?

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si