This is a function of the EGR system and pcv system. Wifes '08 ML320CDI was that bad times 10 with 115k. Positive Crankcase Ventilation mixed with hot exhaust gas = crud. Problem would almost be livable if all it affected was the intake manifold; but the crud that accumulates on the intake ports of the head and the valves is what is really unacceptable.

Will be pulling my manifold in the coming weeks. for an inspection / cleaning. On the ML heads I basically scraped, sprayed and worked the goo out with my fingers while using a shop vac to suck it all out. The manifolds got sprayed with cleaner and power washed.