I just had a similar problem with my 1990 240DL and it turned out to be the fuel pressure regulator located on the forward part of the fuel rail in the engine compartment in conjunction with the mas air flow meter. Both were repalced and now the engine idles normally.
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Sounds like a sticky throttle if you can't keep idle. It won't hurt to try.... you can try a quick cheap trick by spraying a little carbon remover (buy at any auto store i.e. autozone, oreiley's) inside the throttle, open the valve, spray just small amount, close it back up and rev it a few times and repeat. It removes the carbon film after long use. fixed my idle issue in my cars. If its really bad you might have to clean it thoroughly but i highly doubt you have to do that.
you might try cleaning the throttle body first. Buy some good Throttle body cleaner and a old tooth brush take off the air intake hose to the engine, have a friend hold the gas pedal to the floor with the engine off and spray and scrub the throttle body good replace the tube crank it up and rev it up a little to clear out the cleaner this may fix your problem
its very important that the throttle body especially around the throttle blade be spotless clean or they will stall every now and then. then progress to refuse to idle at all. take off the rubber air intake pipe from the top of throttle body [has 3 torqes screws] open throtle by hand and look for carbon/grease/soot etc were the throttle sits when closed. clean with toothbrush and some caeb cleaner get all black off and dont forget the blade, both sides and the edges. scrub then rinse with carb cleaner untill shiny.spray some cleaner in the hole that is on top off the body to. that is your idle air controll motor port.put back together.it will be hard to start at first so hold gas pedal half way down untill it starts then slowly [real slow] let foot off untill you are no longer pressing it. put in reverse and let it idle there a few seconds then do the same in drive. that should do it. sometimes it may stall a couple of times when driving dont hold foot on gas just let it die then restart this allows the computer to readjust its idle stratagee
Sounds to me like the engine in fact is getting gas when idleing, but not getting enough air. Therefore my 2 cents are on you IACV (Idle Air Controller Valve). It's a small steel cylinder suspended in a rubber piece on the front of your throttle body, and its job is to supply the engine with air when idleing. This is a necessity since the throttle body valve is closed when the gas pedal isn't pressed. Take out the IACV and clean it thoroughly, also make sure that the air going from it is not hindered by any gunk or deposits on its way to the intake manifold. (You could have someone hold their hand on the IACV when you take your foot of the gas pedal. If nothing happens it's probably dead or stuck. If something does happen there is still a chance that the air is hindered somewhere).
The idle air valve (located below the throttle body) provides the control for constant idle speed. In order to set the correct air valve opening to achieve constant idle speed, the fuel injection control unit uses information from the air mass meter, throttle valve switch and the ignition control unit (RPM signal) to regulate the output current to the idle air valve. This means that the idle valve is not affected by air leaks or a jammed throttle valve.