Porscheteck9(*) and all of the rest of you wonderful PROBLEM SOLVERS!
'kilyab' has been doing a bit of traveling, and since I entered my reply last in which I declared my 560SEL rough idling problem solved, I have not had opportunity to stop and read the APPRECIATION FEED BACK sent to me!
(*) entered a query of how the replacement of the motor mounts 'solved' my rough idling problem. As the 'rough idling' started developing, this would occur when the engine was at idle, more severe when the engine was cold, and, particularly so when the outside temperature was 'cold'. The engine, idling at about 700 rpm would run rough, and, curiously cause the idle to vary slightlly up and down with the tach showing idications of a fluxuating range of 550 to high 700's - similar in nature to mis-firing, bad plug(s), etc. If the engine rpm was accelerated to, say about 1500, the engine became smooth. Also, I originally reported that the engine ran smooth at driving speeds.
The rubber mounts were, in their worn state, more hard when cold temps prevailed, and, also in their worn state, less flexible, thus proviiding for engine shaking, simulating a rough idliing problem. Obviously, when the rpm's increased, the engine smoothed out. This idling problelm somewhat mirrored other low rpm rough idling problems as had been experienced by the various respondents. I WAS REALLY SURPRISED AT HOW THE PROBLEM DISAPPEARED ONCE THE MOUNTS WERE CHECKED, FOUND WORN (ABOUT 50%) AND REPLACED. My 560 now runs SMOOTH AS SILK, AND REMAINS A JEWEL in our stable!!!
kilyab

old thread but interesting and relevant.

86 560SEL

was at dealer today talking to shop foreman.

mentioned that my car kinda shakes as if bad idle and could he absolutely check for bad fuel distributor as that's what i'd been told was wrong by another mechanic.

he looked around, listened to the tail pipe while i revved the engine... he said there's a ton of stuff i'd check long before assuming the fuel distributor.

then he opened the hood and looked around... then looked with his flashlight and said.."you're motor mounts needs replaced. the right side is sitting on the frame." he said.. you'd be surprised what new motor mounts would do. also should do the transmission mount too while you're at it if it's never been done after all these years.

so.. how hard are motor mounts to change? can you do it in your garage or do you really need a lift?

rectified?
 

Yes it's very nice to know how the fault was rectified.Also its an appreciation of all those who contributed their technical expertise to resolve the issue. Do let us l know !
Thanks

I've had a rough idle on my 560SEL for years, around the same mileage as yours. Heard all the crazy, expensive suggestions: fuel distributor, injectors (which I replaced to no avail), and so on.

The real culprit: Not expensive. Engine mounts had collapsed. Start there.