I assume you are referring to a 202.089 chassis vehicle. Of course it could be many things but it could be the EGR valve is stuck open or the EGR control valve is permitting vacuum to reach the EGR valve when it shouldn't. I had this occur on an ML500 (same basic part design as yours) where the control valve stuck open with poor idle quality/misfire. Above idle it ran fine. To diagnose remove the vacuum hose that connects to the EGR valve to check if the situation is corrected. If it is, the control valve is defective or the control signal to the control valve is defective. Reconnect the vacuum hose and disconnect the electrical connection to the control valve---if the situation is corrected the signal to the control valve is at fault. On the other hand, if you take a length of hose and attach it directly to the EGR valve and apply vacuum you can determine if the EGR valve itself is stuck open and causing the problem. BTW, it takes very little vacuum to activate the EGR valve and destroy the idle quality. If it is the EGR valve or control valve, Mercedes sells it as a unit and it is wise to buy all the ancillary stainless tubing, ferrule, nut and gaskets that are associated with it. The flexible corrugated tubing is very easy to damage and you will not be able to remove the ferrule from the tubing if you purchase only the stainless tubing and gaskets which will delay the repair until all of the parts are obtained from the dealer. Good luck.

Mark