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Old 04-11-2005, 06:10 PM
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Join Date: Apr 2005
Posts: 1
Question 1987 420 SEL very high idle problem.........

Hello everyone, brand new to this forum........

I am hoping someone can help me. I have looked in this forum for "idle control" and found a lot of posts and so now I am not sure how to go about this.

My 87 benz with 104K has a way too fast idle, it's been like this for a few months, but it's getting to be bothersome and I know it's not good for the tranny or the brakes. On starting the car, the idle is as high as 12-15, then after a few minutes, it goes to almost 20!~ Then, it takes about 30 mins of driving, sometimes a little less for it to all of a sudden drop down to a normal 600-700 and then it stays there. Next day, same problem when starting cold.
If I leave the car for 20 mins or so after it's returned to normal, the idle goes back up again to 20 and then it takes 10 15 mins for it to return to normal again.

I saw on this forum search and on the internet, that there is an idle control valve and a idle control a few other possibilities. I would like to save the money and do it my self instead of paying the high price of a mechanic to do it. I am not that mechanically inclined but have done some simple work on my other cars. I was thinking about buying a CD repair manual set on Ebay for this, and following the instructions..... any reccomendations or any suggestions on how to go about it and what might be the problem with this idle?

Thanks very much,


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Old 04-11-2005, 06:48 PM
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Join Date: Mar 2004
Posts: 758
You might check to confirm that your car uses an auxiliary air or slide valve like other V8s. I think it does. These operate thermostatically to increase the air flow during cold starts. As the engine heats it closes off to reduce air flow so that the idle slows down. These valves can get sticky at times and the speed stays up if stuck open, or always stays low if stuck closed. It is at the top front of the engine, has a pair of large diameter rubber hoses running to and from it to the manifold. There is also a large brass slotted screw near by for adjusting the idle speed.

Search the forum, you'll find several posts on this.


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Old 04-11-2005, 07:27 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,207
The first thing I would check is the throttle valve switch to verify that the idle contacts close when the throttle is in the idle position. If the switch misbehaves the ECU will not go into idle stabilization mode.

A flaky switch is usually just a matter of doing a thorough cleaning on both sides of the throttle valve shaft and checking the elecetrical connector on the TVS pigtail for good contact with it's mating connector on the engine harness.


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