My 1980 240D automatic has a quick buzzy shake when idling in gear. In neutral or park, it's quite smooth (as it is when driving), but as soon as you slip it into reverse or drive, it shakes, especially when cold.

So far, I've done the following:

*diesel purge
*new fuel filters
*valve adjustment
*new motor mounts
*new idle control cable

Each of these has helped a little, but the shake is still very much there. Cranking the idle control way up does make it smoother, but I'd rather not have the car idling high all the time, if possible.

I'm planning on putting in a new transmission mount this weekend. Is that going make much of a difference? What else should I try?

With it at idle crack loose each hard line one at a time to hopefully isolate the cylinder thats different. Crack the line loose at the injector. Its much like shorting out a plug wire on a gas vehicle. Once cracked, there should be a noticable rougher running engine. Move down the line till the one you loosen doesn't affect the idle as much. Thats the cylinder thats acting up. Investigate for partially plugged injector, tight valves etc. If all seem the same then look for something attached to the engine thats also touching the body or frame, exhaust collapsed mount, air cleaner housing, etc.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

Assuming this is an older car and you have no maintenance records. How about checking for chain stretch, chain guides and tensioner and injection pump timing? This proccedure also covers cam timing as well. Or do you know those items where addressed at some point in the last say 60k miles? They should be checked anyways if you have no knowledge about your car even if no obvious trouble exists.
Next would be to check the injectors by removal and having them checked for pressure, spray pattern and leakdown. I might even squeeze a compression check in between the two above items.
Make absolutly sure you are not getting any air into the fuel first of all. The idle will be rough if you see a small stream of bubbles being processed through the primary filter for example. The heat on rubber hoses has to be hard on them where you live. Or duplicate the setup if you directly fed the injection cleaner into the system only using diesel oil in the container to get absolutly clear of any air leaks as a source of your problem. You can also do an injector line loosening check as described in the last post or so to establish if all cylinders appear equal as an early step as well. I think you will find it one way or another.

I think the tranny mount will do the trick. Also how long ago did you replace motor mounts? It pays to retorque em after a year or two. Among Vintage MB's the owners manuals listed a bunch of nuts'n bolts that require tightening each 25k miles or so. On this list were motor mounts, steering gearbox mounting bolts, subframe bushings and a few others that i forget. This refers to 111's, 114's but the logic would be the same.

Thanks, everyone. I just finished putting in the transmission mount. It helps a little but the shake is still there at idle under load.

I'm starting to wonder whether that's just how it is with an older 240 auto. If it had a manual transmission, the engine wouldn't be under load at idle and thus wouldn't shake.

I'll try cracking the injector lines next. Perhaps that will shed some light on things...