I don't know what's up with these cars man? My car, an '85 300D Turbo, has been that way since I had it. Runs hotter (about 85-100 degC) in the winter and normal hot (stays and holds at 90 degC) on summer days, doesn't matter whether interstate or city driving, uphill or downhill. But during summer, the gauge doesn't fluctuates as much as during cold weather. Right now that it's beginning to get cold here in the South, especially in the morning, my temp guage would rise up to 100 deg then drops to 85, then stays at 85, then gradually creeps back to 100, then drops back to 85, and so on, at any driving conditions, highway or stop and go.

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'85 300D Turbo - CA Version