My 1985 BMW 318I runs great but doesn't idle down after warming up. It will idle down and back up.
Sounds like the throttle pull-off or idle control thermostatic piece (not sure of the name on a BMW) is bad. Typically there is a small thermostat-like device which, as it heats up, lowers the throttle to curb idle instead of the high idle on start-up.
My car does the same thing except sometimes it will rev up and down and the gages go all crazy, i was told it could be a idle injector valve that was dirty or faulty, but it only does this once in a great while.
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