Hi. I have a 1979 300SD which I've owned since the winter. It runs great but during these hot summer days it starts to overheat. It has to be about 90 degrees and above for this to happen. Otherwise it runs just fine. I've also noticed the temperature gauge gets pretty high when i'm driving upgrade but on the way down it goes back to normal.
Do any of you know what the problem might be? Thermostat? Radiator?
Thanks.

Any coolant leaks in the heater core? Start with the easy stuff, replace the radiator cap.

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