My 1988 740 Turbo wagon was running fine until a week ago. The engine idle spiked from 900 RPM to 1500/1800; 1500 cold and 1800 warm. The mechanical linkage is clean and lubricated; no problem there.

Punching the accelerator pedal drops it to 1100/1200 for a split second, and it then jumps back up to 1500/1800.

The idle drops to 1100/1200 in between gear shifts for a second, and it then jumps back to 1500/1800.

Two or three times in the last week, it dropped back to 900 momentarily, while I was stopped for traffic or a light. This is/was totally arbitrary, and it stayed at 900 RPM for just a flash.

My experience/intuition tells me this is probably caused by one of those obscure sensors that costs hundreds of dollars.

Which one???

Thanks, Dennis Lalancette

There is a threaded plastic plug with a knurled head on the bottom of the throttle housing; idle speed adjustment. How far into the housing should this be engaged? Should any of the threaded portion be exposed?

Thanks in advance!

You could have one of several problems:

1. Connector corrosion leading to misreading sensors. See the FAQ under "performance" for unexplained driveability problems for sensor corrosion.

2. Vacuum leak: check intake manifold gasket and all vacuum lines.

3. Is it performing very poorly on startup? Check the engine coolant temp sensor (to the ECU). If this malfunctions, it enrichens the mixture.

4. Failing IAC valve.

This list is in order of cost. The ECT sensor is around $60 plus about an hour and a half of skinned knuckles.