On a 1989 420 SEL, I had a high idle after the engine warmed-up. The idle was 1,000 RPM after warm-up. On start-up it was 800, but after a 40 min. drive, of which 30 min. was at 100 KPH, the engine would idle at 1000 rpm in neutral.

Problem was solved by replacing the idle control valve. Now the idle in neutral is 650.

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1989 420 SEL

I am trying to solve a similar problem on a 560sel. Did your Idle Control tested as BAD therefore replaced it? How did you isolate that it was the idle contraol value that was causing the High idle?

thx

I had similar issues with my '88 560SL. You can see if the valve has any effect by unplugging it while the car is idling. When I did that the idle speed jumps up to over 1500 RPM. Chances are good if it is functioning at all that cleaning will help fix the problem. You can also test it by briefly applying 12V to it and listening for a click.

If the valve functions you might first try cleaning the valve by removing the valve from the engine and soaking the end of it (up to just past the outlet on the side) in gumout or other carb cleaner until the valve comes out clean. I hooked up a power source to it while cleaning to cycle the valve which loosened up more gunk. Sometimes you need to change the solvent several times. Lots of varnish builds up in these over the years.

In my case, it was not just the IC valve which was the cause of the fast idle but was the seals around the injectors and the o-rings at the base of the injector holders which were leaking air. I landed up replacing every part from where the IC valve attaches to the plenum through the 8 injectors and now the car is well behaved at 650 RPM at all times

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz