I have posted about my car's high idle problem in the past, but I just wanted to give everyone an update. The fix for my car turned out to be the Idle Control Unit. I knew that there would be some benefits to getting this repaired. I was just hoping that it wouldn't destroy the transmission before I was able to get it fixed. I always cringed when I had to put the car into gear. Well I cringe no longer, and to top it off my gas mileage is awesome. With a fill up on Monday, I have driven 296 miles and still have a third of a tank of gas to go. This is great when you consider that I was only getting between 225 to 250 miles to a full tank before the repair to the ICU. The 500SEL is not econo-box by any stretch of the imagination(Thank God), but it sure feels better to know that I won't see the gas pump as often.
The 296 miles are all highway miles running between 70-80mph.
If I have said it once, I will say it again.... This is a great car.
Thanks to everyone for all of your help in diagnosing the problem.
You'll prob. find that once you pass a 1/2 tank it will go a bit quicker then the 1st half..every single gas 126 I have owned has been that way.
Seeing 19-22 mpg is not unheard of in these cars. My record was a hair under 24 w/ my 560SEL.
The idle control unit is under the passenger side floor panel.
Jonathan
Jonathan,
Do you know if a faulty idle control will cause oscillating idle? In P or N (no load) idle will "rev" on its own from 600~800, but in D (with load) idle's fine.
Current problems are CHECK ENGINE LIGHT on, shakey motor at "no load" idle, and unable to pin point any vaccum issue. Plugs and wires are very new and was checked ok.
I tried to re-solder the points inside the idle control unit, but no help. When unit is flexed left and right, idle will rise to ~900rpm.
I plan to change o2 senor this week, do you think it'll solve my problem? My mpg/miles for tank had raised from 270mi to 320 w/ 20 gal fill, same commute and after check engine light came on...
IMO, it is vaccum too. Maybe I'll have to look harder, but I really don't know where to start except for the water trick over injectors.
I noticed the economy gauge is opened to the right a little, more during the slowing down let say from 40mph to a stop.
I'm running superplugs w9dc gapped .38, o2 sensor was replaced 15k ago, but it's probably shot from improper adjustment on the mixture. (way rich) I think the mechanic had done so to compensate for a plugged fuel port in the FD. (FD is new now)
Motor mounts are new 15k ago, and never check compression.
MPG is so much better now... 
Linear Mode
