For 1990-2009 cars

### Thermostat Inspection and Testing Procedure

If the thermostat sticks open, the engine warms up slowly. If the thermostat sticks closed, the engine will overheat.

Inspect the thermostat. Test the thermostat using the following procedure:

Notice: When adding coolant, use DEX-COOL® coolant. If silicated coolant is added to the system, premature engine, heater core or radiator corrosion may result. In addition, the engine coolant will require change sooner-at 50 000 km (30,000 mi) or 24 months.

1. Drain the coolant to a level just below the thermostat. Refer to Draining and Filling Cooling System .
2. Remove the coolant outlet.
3. Remove the thermostat.

• | On the 3.8L (L36 & L67) engine, refer to Engine Coolant Thermostat Replacement .

• | On the 4.0L (L47) engine, refer to Engine Coolant Thermostat Replacement .

4. Unless the thermostat is obviously faulty, test the thermostat before replacing the thermostat using the following procedure:

4.1. | Place the thermostat and a thermometer in a container with a 50/50 solution of water and ethylene glycol antifreeze.

| Important: While heating, do not rest the thermostat or the thermometer on the bottom of the container. This causes the thermostat and the thermometer to be at a higher temperature than the solution.

4.2. | Place the container over a heater.

4.3. | Agitate the solution in order to maintain a uniform temperature of the solution, the thermostat, and the thermometer.

The thermostat valve should start to open at the rated temperature. The thermostat should be fully open after the temperature has increased 15°C (27°F).

A used thermostat can be about 6°C (10°F) above or below these settings without adversely affecting the thermostat and you do not need to replace the thermostat. If the thermostat does not operate at the temperatures specified, replace the thermostat. You cannot adjust the thermostat.

• | On the 3.8L (L36 & L67) engine, refer to Engine Coolant Thermostat Replacement .

• | On the 4.0L (L47) engine, refer to Engine Coolant Thermostat Replacement .