Volume of air is the determining factor on engine speed. Your car uses a feedback controlled idle that uses the minimum engine speed to accomplish idling in various conditions.

You have too much air getting in the motor. One hint is that a dead electrical control of the idle valve leaves it full open which lets enough air into the motor to run around 1800rpms.

Idle control is a pulse width modulated control of the valve. Constant frequency, varying duty cycle. The best valiant attempt to shut the valve will look like a 70% on duty cycle.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician