Thermostat Test

To check thermostat action, watch the coolant through the radiator neck. When the engine is cold, coolant should not flow through the radiator. When the engine warms, the thermostat should open. Coolant should begin to circulate through the radiator. If this action does not occur, the thermostat may be defective.

There are several ways to test a thermostat. The most common is to suspend the thermostat in a container of water together with a high-temperature thermometer (fig. 6-16). Then by heating the container on a stove or hot plate, the temperature at which the thermostat begins to open, as well as when full open, can be determined. If the thermostat fails to respond at specified temperatures, it should be discarded. Specifications vary on different thermostats. For example, a thermostat with an opening temperature of 180°F to 185°F, full-open temperature is 200°F to 202°F. If the test is satisfactory, the thermostat can be reinstalled.

A digital thermometer can also be used to check the operating temperature of an engine and thermostat. Simply touch the tester probe on the engine next to the thermostat housing and note its reading. If the thermostat does not open at the correct temperature, it is defective and should be replaced.

The use of a temperature stick is another way to test a thermostat quickly. The temperature stick is a pencil-like device that contains a wax material containing certain chemicals that melt at a given temperature. Using two sticks (one for opening temperature and the other for full-open temperature), rub the sticks on the thermostat housing. As the coolant warms to operating temperature, the wax-like marks will melt. If the marks do not melt, the thermostat is defective and needs to be replaced.