I have a 1991 560SEL - 115,000 miles on the clock - with a rough idling problem. Car has been tuned, new plugs (old ones appeared to be 'clean'), cylinders checked for compression (all readings were 165+), car does not burn oil. When engine is started, it 'rumbles' as though it has a 'miss' on a cylinder or two; increase the rpm to about 1500 and engine smooths out. When the engine is warm, the 'miss' is still there, but less intense, less even more so if it is in gear. On long trips, mileage is about 18.75 mpg - using cruise control at 74 mph.
MB shops have suggested to me that the fuel injection management unit (my term) which sits on top of the engine may have to be replaced, and it will be EXPENSIVE - say about $2,400 for the BOSCH part, plus installation.
Could this be my problem? Where else might I look. I need help on this, as we really like this machine (it is in beautiful condition), Many, Many Thanks to any answers/comments.
kilyab

Thank you so much for your reply. Curiously, no one has mentioned or suggested testing for a possible vaccuum leak - intake. Understanding this is 'Greek' to me, but I know to whom I will be referencing your suggestion.
If this works, I will certainly owe ShopForum BIG TIME.
kilyab

Along with the smoke test,you will wish to find out if your EHA is ajusted to the proper setting,the on off ratio too.JNT

another sad and common expense to be spend is the fuel distributor.

Avatar, Just in Time, and Cool! Thanks a 'heap' for your responses. I will need to get my 560 from the NY-Long Island location to southwestern PA (near the Pittsburgh area) where I will pursue the various suggestions in an effort to resolve this rough low rpm engine running problem - idling. I will definitely keep you informed. It is now Monday, February 12, and I plan on visiting the MB 'oldtimer' mechanic with these suggestions, although I will not have the car with me until next week. Of course, this depends on the predicted 'NorEasterner' storms due the New England / Eastern NY / Long Island area this coming week. . .end.
Continued appreciation.
kilyab

I have several post on the repair of the intake system and the EHA,as well as the setting of the potentiometer,all tasks I did as a first time repairing a MB.look them up they have pics and it may help you with your mech. Good luck JNT

By the way just because it is so easy to over look,we do at times look past the easiest thing to check! I know I might upset you but not my intentions,did you check the plug wire to see if they might be resisting the flow of current to the plugs?This will cause the very problem you describe.JNT

560SEL Idling Problem Update: Continued appreciation for the suggestions - particularly from Avatar, JNT, and Cool. I have spoken to the MB mechanic and, sure enough, he had considered doing the smoke test on the intake to check for vacuum leak(s). Unfortunately, I have not been able to get my 560 to the place where your suggestions will be reviewed. The weather is the N.E. has been lousy, and I have been driving my Land Rover. Hopefully next week (02/26), I will be able to take the 560.
Will be back to you with results.
Continued appreciation.
Kilyab

JNT, Avatar, Cool, and a GREAT BUNCH of OTHER RESPONDENTS to my NEED regarding the IDLING PROBLEM on my 1991 560SEL - Car is fixed!!!! Here is what happened: Checked and have had tested ALL recommendations provided, smoke test, plug wires, plugs, compression, EHA and potentiometer settings - everything checked out O.K. However, I read another rough idling problem at the forum, and it was mentioned that THE ENGINE MOUNTS (I think this is what they were called) BE CHECKED FOR WEAR. Instructred my mechanic to do this, and I was assured that it would not be the problem. Well GUESS WHAt!?!, the mounts were approximately 50% worn on the right side and about 60% on the left side. I had the new mounts installed, and the idling problem is NO LONGER. I owe you GUYS a HEAP OF THANKS.
Forever Grateful!!
kilyab

It is always great to get the feed back from those who have found the problem,and share there experience and knowledge with those who come in after them,this is just the thing needed.JNT

I have been reading your thread with much interest. Glad you got this worked out at minimal expense. It is often the case that the collective wisdom of the members of this forum is priceless in resolving a problem.

RML

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How did the worn, (squished?), motor mounts affect the idle? I am also curious about the "smoke test" mentioned above. Apparently one post is missing or has been deleted from this thread. TIA!

nice to know you had your ride fixed I truely enjoy the ride of the 560 SEL their great cars with only a few problem as they grow old, its easy to maintain. Have a great day.

The motor mounts did not effect the idle,what happens is that as your mounts fail,They no longer hold the power plant firmly in there grasp,this allows the engine to move around more than when the mounts are new,thus a shake sets in.Kind of like holding a glass of water in your hand and then resting it on the counter less ripples. The smoke test is used by people who like I do not wish to spray flammables on hot engines,the use of smoke is just another way to pin point vacuum leeks. Vacuum sucks,by its very nature,so smoke will follow the leek.I use an old bee keepers smoker and a length of tubing to put the smoke in an area I desire,try it,it is so basic science.JNT

Yes you can and they do put the system under pressure,I don't.JNT