There are several ways to test the opening temperature of a thermostat.

One method does not require that the thermostat be removed from the engine.

• Remove the radiator pressure cap from a cool radiator and insert a thermometer into the coolant.
• Start the engine and let it warm up. Watch the thermometer and the surface of the coolant.
• When the coolant begins to flow, this indicates the thermostat has started to open.
• The reading on the thermometer indicates the opening temperature of the thermostat.
• If the engine is cold and coolant circulates, this indicates the thermostat is stuck open and must be replaced.

The other way to test a thermostat is to remove it.

• Suspend the thermostat completely submerged in a small container of water so it does not touch the bottom.
• Place a thermometer in the water so it does not touch the container and only measures water temperature.
• Heat the water.
• When the thermostat valve barely begins to open, read the thermometer. This is the opening temperature of this particular thermostat.
• If the valve stays open after the thermostat is removed from the water, the thermostat is defective and must be replaced.
• Several types of commercial testers are available. When using such a tester, be sure to follow the manufacturer's instructions.
• Markings on the thermostat normally indicate which end should face toward the radiator. Regardless of the markings, the sensored end must always be installed toward the engine.
• When replacing the thermostat, also replace the gasket that seals the thermostat in place and is positioned between the water outlet casting and the engine block.

Positioning a thermostat into an engine.

• Generally, these gaskets are made of a composition fiber material and are die-cut to match the thermostat opening and mounting bolt configuration of the water outlet.
• Thermostat gaskets generally come with or without an adhesive backing. The adhesive backing of gaskets holds the thermostat securely centered in the mounting flange, leaving both hands of the technician free to align and bolt the thermostat securely in place.