something is seriously wrong there. I would replace the thermostat, that is easy and there have been cases where a new thermostat turned out bad.
Then look into the radiator, etc.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!