Disable one injector at a time when the engine is running rough at idle. It may be a cylinder is dropping out. Or on a long prolonged rough idle read the temperature of the glow plugs looking for one that is much cooler.

A milli volt reading of the voltages at the harness connection will find it if there as well and is more sensitive at locating it than a temperature scan. It is just one other possibility. It is also a very fast test.

Since this is a 3.5 engine this should be checked. I hope it is not what it may be. That still does not mean it should not be checked. You may be describing the onset of a bent rod symptom. .

Bob I have limited ability so I tend to resort to simple tests. If when running rough you provide 12 volts to the glow plug system and the engine smooths out it would be semi conclusive to me that a bending rod onset is a probable scenario. You have to add the voltage in some way that enables a heavy normal current flow to the glow plugs. Technically you could do them one at a time. This may have the benefit of locating the offender as well if there. The nice thing about this test if it does not stop the issue it may mean it is something else.

I do not know where the air intake is for the air filter on this model. Is there the slightest possibility you could have experienced some ingestion of water? These engines do not have strong connecting rods. Well they do but not to deal with a design situation Mercedes introduced in my opinion. Actually there may be two of them but that is just my opinion.

Nobody ever asked me why they bend so I just observed all the speculation. I have held my own opinion for a long time but rather than get into a situation of what I think the possible cause is and the resultant derision initially have kept it to myself.

I assume normal idle was in the 600 area before and the idle remained good. If it went lower than it was earlier is another scenario in itself. I added this as an afterthought when reading the following post. I just assumed and that is dangerous that the idle speed was as it always was until the roughness shows up.