Mine used to stay at or near 80 and now goes near to 100 on hot days/highway or uphill, coming back down if I freak and put the heat on to cool it or simply decrease the stress. It's definitely running hotter than it used to overall. Shall I change out the thermostat? Or flush and fill with MB-recommended fluid. Haven't attended to thermostat in the time I've owned the car. Is it warning me?

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1980 300SD 220,xxx
somewhere, nc