This is on the 79 300TD.

Lately, if I sit still or even drive slowly (30mph and under), my temp gauge rises towards the top after a minute or so. If I get back to highway speeds it will drop back to normal within 10 seconds or so. Since it has been getting chilly here, I have used the heat and the heat (or lack of) corresponds as well meaning at idle it starts blowing cold while the temperature rises.

I have the servo unit and it works properly (seemingly). Of course I will check the Aux water pump but that should not cause over heating, especially in 40F weather, correct? So perhaps something is blocking/restricting the flow? What else could cause this? Air? I have no overflow tank so I'm not entirely sure how full the radiator should be. What do you think about the main water pump as a cause?

I checked the fuses and there were no obvious ones out. I guess I'm not sure which ones would be directly related to this issue anyways.

I am hoping to park the 79 for the winter soon. I'm waiting on the tranny for my 94 6.5 so I can get the put in and drive that for the winter. Once I park it I plan on doing the maintenance. I will flush the coolant, check the Aux pump function and will check out the main water pump.

Thoughts or anything else I should check?

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD