Here's what I've done.

This is the new (to me) 97 GLT I bought (I call him "Pale Rider").

I changed the timing belt/water pump, and filled the reservoir with water (to check for leaks). Started the car and initially the temp gauge didn't show any life.

I've checked to see if it's a head gasket issue, and I believe it is not a head gasket issue. There are no signs indicating that. Oil looks perfectly fine, the reservoir doesn't have a sheen or any sort of gunk built up, the exhaust is normal in color, there is no signs of leakage at the gasket and the car runs perfectly fine.

I've let it run for a while with the radiator cap off in order to "burp" the system. So I know the thermostat functions. Whether or not it functions properly, I don't know.

There is heat blaring out of the vents, but the engine itself doesn't feel too hot. The hose from the radiator to the thermostat gets hot to the touch and the water cycles through the engine into the reservoir.

But the temp gauge stays low (like 5:00 if the face was a clock).

My hypothesis is that it's not air in the system as much as it's possibly a bad ECT and or thermostat.

Does that sound like solid thinking?
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1985 245. 246K Gone..but not forgotten. Sold in 2013, found 2018! Still running on the road. Of course. 1993 Volvo 850 soon to be gone