When engine is hot the idle increses and decreases every 3 to 4 seconds. This happens with the a/c on or off.Sorry this is an 89 420 sel.Car has all new plugs and wires. Any suggestions?

I am assuming that it runs fine when the engine is cold. It sounds like a fuel problem. Start by checking the fuel accumulator.

Vin

I am having the same problem. It seems to be the throttle body switch. I know that my switch is not making good contact when the throttle is in the idle position. When the contact opens, the idle controller stops regulating the idle speed and it creeps up. Then due to vibration or some other phenonemon, the switch makes contact and the idle controller regulates the idle properly. If I quickly press the gas pedal and quickly release the idle returns to normal, sometimes, or not.

You can check by using an ohm meter to measure the resistance of the contacts. The plug for this switch is located just under a PCV hose very close to the cold start injector valve. Move the hose and unplug the connector. There are three wires. Center is common and the one contact is made then the throttle is fully closed, and the other when the throttle is fully open. Make sure that you get a good steady low resistance reading at each throttle position.

If thats not it you might have problems with the idle control valve. Its that cylinder shaped device just to the left of center on the top of the engine. It vibrates when the engine runs. They get gunked up with goo and need a cleaning from time to time. I would put money on the switch if I were a betting man.

Good luck. Let me know what you find.

__________________
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78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
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VINAMG Where is the fuel accumulator and what does it look like?DPETRYK I will check the throttle body switch today thanks guys.

It's attached next to the fuel pumps, rear passenger side. Run the car for a while. Then disconnect small rubber hose on accumulator and check if any fuel comes out of the accumulator. If it does, you have a bad accumulator.

Vin