You may need a new thermostat but at 75 deg C (167 def F) you should be getting plenty of heat from the heater. Often the monovalve diaphragm fails allowing coolant pressure on both sides. It then opens irregularly or not at all. It is a cheap fix and you can get the part at Fastlane. Good luck.

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Tom Savage
Vienna, WV
1984 300D Euro
1995 S320