First thing to check is the idle control valve -- may be sticking open.

Usually, that causes stalling when coming to a stop, however, as it sticks closed more often than open. Worth a try, anyway.

Otherwise, your overvoltage protection relay may be bad, causing the idle control circuit to fail (high idle).

I would also verify that the throttle position switch is working so that the computer knows it's supposed to be idling the engine!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!