Limp mode is in reference to Electronic fuel/throttle/or trans controls. In the 1988 300E there isn't a limp mode on these items. The only RPM governor is in the fuel pump relay on these engines.
Build up of carbon is common on these cars WHEN the valve guides are worn or the valve guide seals are too "hard".
MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)