Check the MONO valve for having voltage to it as well as removing its lid and verifying the rubber diaphram within it is not torn. Do this with a cold engine, lifting the lid will only loose a small amount of coolant. Its located to the right just behind the battery looking from the front of the car. If torn buy a MONO valve repair kit. Loosen it from the firewall to access all four lid screws.

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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.