This buildup is from a couple of things. Improper oil changes and improper crank-case vent. On the front of the engine going to the other valve cover, there is another hose that vents crank gasses into the vacuum system. The hole gets clogged. Clean it and make sure it sucks a good vacuum.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS