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Fast idle

car has begun to idle very fast ( comes and goes ) ( 1992 240 )

Possibly a faulty coolant temperature sender. Computer thinks the engine needs to warm up. Can you replace it yourself?

Dirty throttle body
Faulty idle air control valve
Vacuum leaks

Open the hood and listen for any obvious vacuum leaks (hissing) at idle
Clean the throttle body
Tap on the idle air control valve. If the idle changes, the IAC is faulty.