Hey guys,

If you own a 560SEL, 560SL or 560SEC etc etc, you have most likely tackled the tribulations of high, wavering, rough or plain bad idle quality. I'm making this thread because this question comes up a bit, and feel I have come up with a great checklist for things to check for new owners of the 1100rpm club, or even vets who have tackled it down to 9 or 800.

First things first:

This generation of M117 have electronic idle control. While the idle is smoother and more efficient, the dependence on perfect vacuum and various valves provides a playground of headaches for the DIYer.

Electrical things that affect the idle are:
-Coolant Temp Sensor. Produces a high idle to about 900rpm. At least it did in my case. $75 at the dealer.
-Idle Control Unit. This little computer is under the false floorboard on the passenger side. I got a second hand one for $20, and turns out it wasn't bad in my case, but now I have a second.
-Idle Control Valve. These get sticky from carbon going through. Gets the idle stuck to about 1100-1200rpm. Disconnect it and the idle rises? It's functional, and worth cleaning. Use carburetor cleaner. I personally sprayed it and achieved acceptable results, but some like to soak it over night to get the coke out.
-Ignition Control Unit. This is the Big Kahuna computer behind the side trim that slides towards the passenger seat next to the aforementioned false wall. Mine was bad and caused stumbling and rough idle on some days. $700 at the dealer, but mine is second hand. Be aware of part numbers-- only some are interchangeable.
-Kick Down Switch-- Mine was alright. I think.

Vacuum things that affect idle:
-All of the vacuum tubes and air injection ports on the surface of the engine. Roughly $200-$250 at the dealer to replace; highly recommended at these cars ages. These include the injection rails, breather hoses and two L shaped ones that connect to the Idle Control Valve.
-Manifold gaskets. Cheap gaskets. Labor intensive R&R. Not for the faint of heart, so I smoked the car after doing the surface rubber. Lo and behold, these were alright. Phew.
-Thermovac switches. There's two, they have two elbows on vac lines each. $35 a pop OEM. Didn't do them yet, but about to.

I hope this helps someone out-- please chime in if you know of other terminology for these things to help searchers or other things I may have forgotten while doing the write-up!


-Carlen

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1987 560SEL (210,000 miles)