The radiator is probably plugged or restricted internally. You might try back washing the back of the radiator surface to remove years of bugs and other debris from the radiator face. Easier with the fan shroud out of the way. Also how old is the radiator cap? If weak, coolant temp will raise. Get a Mercedes brand new cap.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.