I'll check the fan clutch, it's very tight when cold, but haven't checked when hot.

I used the better thermostat at the time, Whalen or something like that, I have a new Mehle one in the box, I'll try it out. I saw on some site, they drilled holes in the thermostat?

I'm not sure the temp gauge is accurate, nor how I'd check it?

Pump belt is tight, about 1/4" deflection when pressed on the middle.

Tank cap seems fine, of course with the waterless coolant it doesn't pressurize as much, only to about 4-6 pounds.

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1983 W123 300TD US spec Turbo engine, with Euro bumpers and manual climate control, and manual transmission.