The ingestion of exhaust soot and oil vapors is a byproduct of EPA legislation and CAFE standards. *ALL* engines have a PCV system dating all the way back to the 1950s and 60s which replaced the "draft tube" of the earlier cars. As a result, you're going to have oil vapor and residue in the intake manifold. Add in EGR and you now have a carbon source present. Add a couple hundred thousand miles, and you now have a bunch of soot stuck to oil residue and it turns into a hardened mass, choking down the manifold's ability to breathe.

This doesn't just affect Mercedes, the modern direct-injected gasoline cars are having a terrible time with the valves getting gunked up from EGR. The concept behind the OM606 PCV design was to try to improve the induction action of a N/A diesel engine by placing the "vents" for the PCV system as close as possible to the valve and at the end of a narrow intake runner. The concept was sound, the implementation not so much.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]