I am a new owner of a 1988 560SL that idles about 700 RPM in drive and over 1000 RPM in neutral. What should the idle be?

That sounds very high. I don't have the specs for that engine, but 650-750 in neutral is closer to where you want it. Someone will have the correct numbers.

According to the workshop manual, the correct idle speed of Park is 650 +100/-50. In drive, it should be 550 +/-50.

To fix this problem could be a myriad of complications (welcome to the M117 engine), but I suggest starting at the idle air valve (top front of the engine under the air cleaner housing) and clean it out with some carburetor cleaner. After that, it could be any of the rubber hoses and grommets in that engine.

I still haven't sorted all of mine out after nearly 4 years of ownership

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1987 560SEL (210,000 miles)

Some may suggest adjusting the air screw. Do not do that. That adjustment is factory set and does not go out of adjustment all by itself. Almost everyone that messes with it gets in trouble.

High idle on our cars is almost always the result of a vacuum leak. Inspect all of the hoses and fittings before doing anything expensive. Rarely, high idle can be caused by a bad engine temperature sensor. Clean that idle valve as suggested then start looking at hoses.

My '83 380SL has an idle controller behind the glove box, looks like a big relay. The connector had come loose on mine and caused an idle speed about like yours. Not sure if the 560 has one though.

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa

Thanks - I will check on these suggestions.

I have a 1988 that is experiencing the same thing.


When cleaning the IAV, do you simply spray it with carb cleaning a few times?

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Current : 1988 R107 560SL

Previous: W123's, W211, w210 E55, W463's, W108's, W111

High idle on the M116/M117 engines is usually caused by vacuum leaks. On my M116 (essentially the same) I had to take apart the whole top of the motor and replace EVERY piece of rubber, especially the rubber rings that seal the two halves of the intake. Once everything is done and replaced, it will idle smooooooth. Mine idles at 500 in drive and you can't even feel it, at idle in park its around 700 unloaded, 600 loaded (a/c or electrical loads)

If your engine has never been torn apart to replace all the rubber parts, its long overdue, they will all be hard as rock and leaking vacuum.

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