Thermostat, Engine Cooling

In order to inspect if the thermostat valve is opening properly, perform the following test:

Completely submerge the thermostat in a glycol solution. The solution should be 11°C (22°F) above the temperature indicated on the thermostat valve.
Thoroughly agitate the solution. Under these conditions, the thermostat valve should open.

In order to inspect if the thermostat valve is closing properly, perform the following test:

Completely submerge the thermostat in a glycol solution. The solution should be 6°C (10°F) below the temperature indicated on the thermostat valve.
Thoroughly agitate the solution. Under these conditions, the thermostat valve should close completely.