JoshMB,

Those cars have a manual idle control knob on the left side of the steering wheel, on the dash, which looks a little like a black plastic, squashed jelly bean or something. You turn it one way to speed the idle up and the other way to slow it down. It is used to make the car idle ok when it is cold. When the engine heats up the engine can race a little when you have it in neutral. Turn the knobin the "slower" idle speed direction to slow it down. I do not remember which way is which but the response is apparent.

Also, check to see if the linkage in the throttle actuator area is clean and lubed. It can get pretty sticky after 25 years if no one cleans and lubes the ball joints in the linkages. This is something I routinely combine with a valve adjustment as you have to disassemble the whole mess to get the valve cover off.

I have made some "custom crafted" parts from popsicle sticks to take up the slack in some of the linkage parts on my 240D (mainly because I cannot get the dealer's microfiche files to agree with the parts in my 1982 240D) and these have been known to change shape after being wetted with Diesel fuel, leading to issues with throttle response (nothing a new stack of popsicle sticks can't fix though). I do not recommend such solutions, but I know if I did it others might also, so look for home made junk in there too.

Good luck and I hope this helps. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)