Checking thermostat

1.9 Liter engine.

I am wondering how to check operation of thermostat on my car. It takes too long to warm up and heat is only sub-par. Considering the thermostat is seated and working properly I have concluded the bypass vapor valve in the housing is not working and allowing too much coolant to pass.

Should it be plugged in the holes or should it be unplugged. When I invert it (off the vehicle) a plunger moves back and forth. I can hear the rattle and actually see it move. I was told that there was supposed to be some kind of plastic plugging some of the holes and that it melts at a certain overheat temperature to allow the coolant to bypass. When I blow into the hole it freely passes the thermostat. When I invert and the plunger moves air will not pass.

How is it supposed to work? No one seems to know and I spent six solid hours searching on the internet.

The temperature sending unit is working properly. It has been checked several times. Thanks for the suggestion however. I am needing to understand the thermostat housing and more important, how to determine if it is defective or not.

I have two available; One of them rattles when inverted, the other has some sort of plug or debris in the hole facing the block and it does not rattle when shaken. The rattling is due to a piston or valve (plastic) which goes up and down when turned over as described Which one should I use?

Thanks, Carl