I have an 83 380 sel and a month ago you would start the car sometimes and the idle would stay at 2000 rpm and sometimes goes back to normal by restarting the car. But now it is doing it most of the time and especially goes high while I am driving.

Isn't it bad for the engine and transmission to be forced at this speed while lets say, stopped at a light? And the worse thing is, to shift the gears you have to hold on for life! My mechanic looked at it and said it is the idle control unit. He said that in the engine there is another part that deals with idle and can get stuck but he checked and it wasnt. Does anyone know why this happened?

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99 C43
98 S420
99 C230K
01 C240

Sounds to me like your mechanic hit the nail on the head. I had the same problem, but mine was intermittent. I fixed mine by resoldering all of the components on the idle speed control module. I've described what it takes to fix it in this post: http://www.peachparts.com/shopforum/showthread.php?threadid=16110&highlight=solder+and+idle+and+speed

first check the OVR. it fixed my problem.