Lots of food for thought here .

Depending on how long ago they were both done I'd begin with a valve adjust then pull the injectors for a pop test and check the compression, if all the injectors popped properly and sprayed well with zero drooling I'd next try bumping the hot idle (yes, it makes a difference) to 800 RPM's and see if that helped .

I had a loping idle issue in my 240D that was greatly helped by bumping the idle up to the maximum, it was long ago and I'd forgotten the specification .

Your comments about it being a worn engine *may* be valid but I've had a few that were simply clapped out yet ran and idled very well with the AC on .

Please keep us in the loop that we may learn from your situation .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Not necessarily. It could be that the linkages are right at that point where it works well, but an added load on the engine pushes it over the edge.

I would at least measure the linkages and make sure they're in spec per the FSM. Easiest troubleshooting steps to do first without hacking at anything else.

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-Typos courtesy of my mobile phone.

The world got weird after I posted about this issue, and it's still not really back to normal. At any rate, I finally worked on the car since I'm having to drive a bit more lately, although I'm still not at pre-pandemic driving levels, thankfully. I rather enjoyed life slowing down a bit, although the cause was a real bummer.

I think a lot of the issue was just deferred maintenance. I did the following work and the car is running much better: changed engine and transmission mounts, adjusted valves, changed fuel pre-filter and main fuel filter, and adjusted hot idle speed to ~775 rpm.

Now I need to wash/wax/vacuum off 2.5 years of dust from sitting in the parking garage at work.

Thanks for the helpful replies. I guess the lesson here, as always, is to keep on top of proper maintenance and start with the relatively simple stuff.

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013