I'm currently on a road trip with my '82 300CD-T, and I'm having problems with it running hot. It's not overheating, but only because I'm able to mitigate the problem by turning on the heat in the car. It looks to me that my coolant is pretty dirty, so that may be the simple answer, but I'll run down the other symptoms in case someone thinks it could be something else.
- Only runs hot at highway speeds. Doesn't overheat immediately, but over a period of time driving at highway speed. I haven't sat in much stopped traffic, so I don't know if it would overheat then as well.
- Running A/C makes the car overheat.
- When it begins to get hot, all I have to do is turn on the heat and temp drops IMMEDIATELY to a safe temp. Will stay that way all day long if heater is on. Air coming from vent is so hot that I can't keep my hand next to it for more than 20 seconds.
- Temp gauge sometimes spikes up and down very quickly, as if something were bumping into it. I would blame this on a faulty gauge, except that it always stays within a certain temp range when it does it (i.e. It doesn't just randomly flop all over the dial.)
Is it just in need of a radiator flush? I just got a new thermostat last week.