My engine idles at around 900-1k rpm, drops to about 600 rpm with ELR unplugged. My understanding is that the ELR is supposed to increase the idle when A/C is running or the engine is below a certain temperature, so it would seem there is a problem with the ELR or one of the components that controls it.

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1985 190D 2.2L 717.411 5-speed
1983 300D Euro OM617.950

I remember my '85 had a high (900-1K rpm) idle in park or neutral after it warmed up.

Not sure if it was due to wear or a misguided previous owner/mechanic trying to artificially raise the idle (strange as the ELR system did function &
actual idle speed ended up being fine), but adjusting the throttle linkage fixed it. I'd start there.

If your idle increased after it warmed up, I have to wonder if something was connected backwards.

My engine actually idles at 750 rpm right after I start it, but stays at 900-1k after touching the pedal. Another thought that crossed my mind is that maybe my ELR "thinks" the car is in park, since it originally came with an auto and now has a 5-speed manual. My old 1978 300D idled really rough when I stopped at a light while in drive, but was much smoother in park/neutral. No tachometer to verify the idle but I'm fairly sure it was a higher rpm in park.

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1985 190D 2.2L 717.411 5-speed
1983 300D Euro OM617.950

It has been over a decade now, but mine just needed to be lubed and adjusted so there was no longer a preload on the linkage.

Early U.S. 190D's (up to a few months into MY85 IIRC) apparently did raise the cold idle speed under a certain temp using a vacuum controlled system, but the later electric system is more to stabilize the idle (at least on mine it didn't appreciably raise the idle per the gauge when cold) and has no transmission input.

I still think you should start by lubing all the linkage ball joints and adjusting it per the FSM before going any further.

As I mentioned in my original post, the idle drops if I unplug the ELR. But the linkage might be related to why it starts at 750 and then sticks at 900 once the pedal has been pressed.
With the ELR disconnected, engine starts at 500 rpm and sticks at 600-700 if I rev it. Idle is also very rough without the ELR.

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1985 190D 2.2L 717.411 5-speed
1983 300D Euro OM617.950