There can be an air bubble right where the water temp sensor screws into the engine. When flushing and refilling the engine with coolant, I remove the sending unit until coolant flows out of the hole.

ALSO: I've tried to 'fix' a non-existent overheat problem for a couple of years now, only - after replacing the temp gauge three times - to find that the Volvo gauges and 'pessimistic' - reading much hotter than the engine actually is.

The solution to an 'overheating' problem is to first determine that you actually have a problem. I installed an after market temp gauge and it read just fine while the Volvo unit was pegged. It is possible to recalibrate your original gauge by gently squeezing or opening up the 'C' shaped expansion chamber on the gauge. I'd have a spare gauge handy in the event you manage to break it.

I also got a non-contact thermometer at Harbor Freight that I can use to check different areas of the engine, searching for hot spots, and even check the radiator tank temperature without having to open it up!

Not to mention being able to check tire surface temps to determine if a wheel is out of alignment, causing the tire to scrub on the road surface and heat up. Then there are the wheel bearing temperatures, etc.

Anyway, I'm glad you found it was only the thermostat. Much easier to replace than the water pump or any of several other things that might be defective.

Steve in California
(Near Death Valley)
1964 Volvo PV544
1965 Volvo P1800s
1967 Fiat Abarth 100-OT Coupe
Etc.