miozzi, I also recommend using Beru spark plug wires and Bosch distributor cap & rotor. You should also replace the distributor cap O-ring seal because it's probably dry/brittle.
The spark plug specified in your Owner's Manual may no longer be available. Suggest calling the Parts Department at your local MB dealer and ask what spark plug they recommend for your 1989 300CE. You definitely want to use a copper core, non-resistor spark plug.
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Fred Hoelzle
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