I had this issue for the past 3 years. Idle RPM most of the time stays at 500rpm. Engine runs "smoothish" with a slight vibration due to low RPM. At times (mostly in cold weather) would go back to 700rpm and run "buttery smooth" but then it goes back to 500. No error codes. I actually took it to the Mercedes dealer for an inspection and they couldn't find anything (although, I have a feeling that all they did was to hook it up to the computer).
So far I did the following: changed fuel filter, cleaned intake when I had the oil cooler gasket replaced, cleaned injectors (running BG245 through them), although computer shows the numbers within specs according to the Mercedes mechanic.
Another thing I noticed, engine starts a bit later than my previous diesel cars, turning an extra 1-2 sec before starting. It never fails to start though.
Other than this car has lots of power and pick up and it runs smooth outside of idle.
Any suggestions what I could do?