But there is a simple way to test a thermostat. The temperature at which the T-stat should open will be stamped into the T-stat. Get a hotplate, a tempered vessel that will withstand the heat, some water, and a thermometer or temperature measure device (like a thermocouple) that’ll measure the appropriate temp. Using a piece of wire, hang the T-stat in the water with the thermometer (or whatever). Heat the water to the temperature indicated and see if the T-stat opens. If the T-stat opens way too early, the engine will run cool.