No platinum plugs in the M119 engine! Copper core only like Bosch F8DC4. Get the basic ignition parts right before you diagnose further. This engine also likes to eat caps, rotors, wires, and coils, which are all sources of missing and poor running. -from personal experience

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1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out