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You should test a thermostat in the following way; read on the thermostat the temperature at which it should open and note this down. Hang it in a container or pan and heat the water slowly. Make sure the thermostat does not touch the bottom. Use a thermometer to read the temperature of the water.
As soon as the thermostat starts to open, read the temperature on the thermometer and write it down. In the meantime, continue heating the water until the thermostat is fully open, usually about 10 ° C warmer. For example, open at 75 ° C and fully open at 85 ° C. Then you read the temperature again and note it down again.
Check whether the measured temperatures correspond to the values stated on the thermostat.
Washing thermostats that have been very hot can (it was before) jam on the shaft / oil seal. In case of a problem, I test them with a clock, then you can measure the opening in mm. according to temperature.
Note: what is not allowed is the thermostat by hand!
As mentioned, the temperature at which it opens is displayed on a thermostat. In direct-cooled engines, it opens at 55 ° C and the thermostat is fully open at 65 ° C.
The Thermostat is in the pan. The Thermostat is fully open, do not let the water boil
