This test requires that the thermostat be removed from the vehicle. Refer to Engine Coolant Thermostat Replacement in Engine Cooling.

1. Suspend the thermostat (1) and a thermometer (3) in a pan in a 50/50 mixture of ethylene glycol and water. Do not let the thermostat (or thermometer) rest on the bottom of the pan because of the uneven concentration of heat in that part of the pan.
2. Place the pan on a burner (2). Heat the solution and measure the temperature of the solution using a thermometer.
3. The thermostat valve should begin to open (depending on whether it is a type A or a type B) when the temperature reaches 88 to 92°C (190 to 197°F). When the temperature of the solution reaches 100 to 105°C (212 to 221°F) the valve should be fully open.
4. If the thermostat does not perform to these specifications, replace the thermostat with a new one.