I think you are forgetting the weather. When you use the heater a lot in these temperatures, the cooling effect of the heater alone can be enough to cool the engine.
This is normal with heavy use of a heater, where there is an unregulated cooling of any engine, as the heater takes hot water from the engine, bypassing the thermostat, this is not a function of the metal thermostat.
The thermostat I use, starts to open at 80 degrees, and progressivley opens more as the temperature rises.
If you use the defroster for the front screen, the engine fan will also be running all the time increasing cooling on the radiator.
The thermostats are tested before installation, and I have yet to come across one that does not open at 80 degrees spot on! So unless there has been some physical damage to the unit, I doubt it will be faulty.
In order to verify this yourself, switch the heater off, and you should see the engine temperature rise.
Getting a feel for the thermostat operation can also be obtained by letting the engine idle and feeling with a hand on the outlet pipe from the thermostat housing. The pipe should become warm as the thermostat opens.
There would certainly be a thermostat in the housing, you would find it hard to miss that one! I am sure also that Lates would have noticed if it had gone AWOL. I don't think you have to worry about that.
Thus, heater off, defroster off. Let us know what you get.
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