Try this:
https://www.startekinfo.com/StarTek/outside/12264/Program/ETM/ETM.pdf

FSM for 91 300E electrical troubleshooting.

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1990 190E 3.0L

Man, that has a lot. I am not exactly sure what that potentiometer is called in the docs, but I didnt find anything that indicated a valid range.

Would any of you great people with an m103 be willing to take the same readings on the throttle position sensor?

Had the exact same problem as you with my 1990 300SE, with the only difference it solved it self after I replaced the head gasket. And here is the weird part: after the first start up with the new head gasket the idle immediately went up to 1100rpm and after being warmed continued to 1800rpm like it always did, stayed at 1800 rpm for a few seconds and I heard a bang, idle immediately dropped to 700rpm. And that's where it stayed, the idle was perfect for almost 1.5 years, unfortunately the high idle has returned on occasion after I washed the engine bay a month ago. But only for the first 10/15 minutes after the first start up in the morning.

Changed or checked every part that could have caused the high idle, nothing fixed it or would even change the idle speed. Just as in your case if I disconnected the icv the idle would drop to 1100 and to 800/700 in gear...

Good luck.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Yeah, I am struggling.

I am going to try and do a smoke test tonight just to be sure of the vacuum leak problem.

To me it seams like some sort of electrical malfunction based on a sensor reading or something. If it was purely a vacuum leak I would expect high idle even on a cold engine. In this case it’s like the ECU is opening up the icv for no reason. Or... there is a leak and the icv is opening to make it what would be a normal idle without one.

I love these engines but there are so many places for a vacuum leak on CIS injection. I can’t complain too much though, it is a 28yr old car and I have had it since 2005.

Ok, I finally found something.

I did the smoke test, and I found one leaking fuel injector port. But then I noticed more leaking from under the air flow sensor housing. I decided to dig deeper. I removed it (getting easy at this point), re-seated the fuel injector and did my smoke test again.

I found a leak on both sides of the throttle body where the shaft comes out and attaches to the throttle linkage and where the position sensor is fixed.

Questions:

Is this normal?

Can this be rebuilt or do I need to find a whole new throttle body?

During my smoke test I had smoke coming out of the valve cover. Is this normal or should there be a check valve in between the intake and valve cover?

There is a PCV valve between the intake manifold and the valve cover, but it only closes when the engine is running and the throttle plate closed. The valve cover and oil filler cap should have a good enough seal to prevent smoke rolling out. The valve cover will fill with smoke during a smoke test due to the PCV breather connecting to the intake manifold.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/07.3-0121.pdf