I just got my 94 c280 ~350 miles ago, this sounds like vacuum problem, but I'm not positive. These gas burner MBs are new to me!! Good Luck, I'm hoping someone that knows more than I will help out.

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If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon