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

I also agree with your assessment and course of action. In my experience 850s are not prone to air pockets or needing "burping" of the cooling system. Luckily the thermostat is easy to access on these cars (and the ECT sensor is right there too).

Twice I've seen ECT sensors fail but both times they failed such that the temp gauge spikes to the red (scaring the crap out of the driver until they realize the real problem). You may find you don't need a new sensor, but it's good to have one handy.

One more thought - check that thermostat box to be sure you're getting a new o-ring seal. Last time I did one I found out that they now sell the seal separately.

Best,
Will

Thanks. I did check and it didn't come with one so I bought one. Luckily they all arrived the same day (the ECT, Thermostat and o-ring). Installed them last night, and filled up the radiator. I didn't start it yet (I didn't get to it until real late in the night).

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1985 245. 246K Gone..but not forgotten. Sold in 2013, found 2018! Still running on the road. Of course.

Thermostats fail in the open position on 850's. That will never allow the engine to reach correct temp and show low. Change the t-stat while your in there.

Thanks for the info. I've purchased a new t-stat and ect just to be safe (should have done that in the first place).
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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