Has to be a bad idle control valve or a big vacuum leak. Idle control valve is under the air cleaner housing, two wire plug and two big hoses. Clean it out with carb cleaner, and replace the hoses if they are hard, they leak badly in that state.

You should get about 1000 rpm on initial startup, dropping to the normal 500 rpm in gear in a couple minutes.

If you have to keep your foot in it to keep it running cold, you probably have a serious vac leak that the feedback system cannot compensate for. Check idle control valve hoses, trasmission modulator line, charcoal canister purge lines, and any other vac lines you can find. Typically, by this age all the rubber connectors are shot, so plan on replacing them all. You can use standard vac hose for the stragiht ones, but the "y" and "t" ones I'd get from Benz or an independent, you really need the proper ones.

You will probably need to adjust the idle mixture after fixing the leaks.

Merry Christmas!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!