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Heat but no Indication

I change the thermostat and radiator cap every other coolant change.
I consider this a part of basic maintenance.

@Bardiferous

No offense, but nobody in the real world actually puts their old thermostat in boil water to test it

If in doubt, a thermostat gets replaced, preferably with a factory part

As @circuitsmith said, thermostats should get replaced every so often as preventive maintenance, because they tend to get stuck as they age

The rubber gasket seal in the thermostat had come unseated and so, yes, it would not fully close. See pics below.

I went with the Toyota brand antifreeze; found the thermostat in the connection on the engine block from the lower hose; and screwed up a bit by putting some green antifreeze into the system but did 3 flushes to clear most of it out before filling up with the red.

Thanks to all contributors. Now, about those squeaky belts…

This is actually not an uncommon problem

As with the belt, I recommend genuine Toyota belts. Had too many problems with noisey aftermarket belts

Congrats @bardiferous , and thanks for taking time to post photos of the offending part.

fyi, @Texases , a regular poster here, told me about a vdo I eventually found, a DVD that shows how car parts are actually made, and there is a segment on how thermostats are made. It’s wild and crazy stuff. You’d have to see it to believe it. If you tried to make one yourself it would probably take 3 or 4 months to tool up, or longer, plus it probably would only work for a week since you wouldn’t know all the exact recipes for the ingredients and dimensional specs, like the special spring steel that won’t corrode, and the thermo wax, etc so think of it this way: If you can buy a good one at your parts vendor for $15 its a very good deal.