The car is a '93 300CE with the M104 engine. What actually happens to change the idle from a value over 1000 rpm on a cold startup to the normal 800rpm after it runs for a few minutes? I couldn't find anything in the archives.
Thanks,
Anziani

'93 300CE 172K
'95 E420 116K

It is looking for a higher coolant temperature with the sensor on the engine, once it has achieved a specified temp, it lowers the engine rpms....by adjusting the idle control and fuel injection etc...the initial 1000-1100rpm idle is to warm up the engine and catalytic converter as quickly as possible, for emissions reasons.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

PawoSD,
How is the idle control physically set? I have a strange intermittent problem that occurs infrequently a few minutes after I start the car leading me to think that the car is reverting back to 800RPM on its own even though we are at 1500RPM
Thanks,
Anziani

Its not physically set, the computer determines the idle speed. Default rpms in park once warmed up are indeed about 800rpm or so....and about 600 or so in drive/reverse.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

And further, the computer takes info from the coolant temperature sensor at the rear of the engine to determine the correct rpm and fuel mixture. If your sensor has been cooked or is old replace it.

it's called a thermistor, big guy in the fuel injection world....it's an interesting little monkey this engine coolant temperature sensor... I replaced mine a year ago along with new plugs and my 1990 300TE has really run like a champ since....that was last September and it's still OK.
It was the plugs and the coolant sensor....it was like $75 out the door, the car has been fine since.....not to say that this little piece of car won't tee me off again shortly...I'm not saying that it's perfect....but I am starting to understand a Mercedes Benz (my 1st) after about a dozen BMWs and
possibly appreciating what they are all about....they (MB) are quite similar to them (BMW) yet different....weird.....anyway..I would change the coolant temp sensor first and go to a general tune up next, starting with a spark plug swap and procede from there...that's what I'd do..best of luck...cliff