GM Service Manual Online
For 1990-2009 cars only
  1. Inspect the coolant outlet and the coolant crossover for cracks.
  2. Check the thermostat using the following procedure:
  3. 2.1. Suspend the thermostat and a thermometer in the water with the thermometer located close to the thermostat.
    2.2. The thermostat must be completely submerged.
    2.3. The coolant must be thoroughly agitated while heating. Do not let the thermostat or the thermometer touch the sides or bottom of the container.
    2.4. Apply heat to the coolant.
    2.5. Record both the temperature at which the thermostat begins to open and the temperature at which the thermostat is fully open.
    2.6. The thermostat should begin to open at 1.6°C (4°F) above the stamped temperature located on the thermostat.
    2.7. The thermostat should be fully open at approximately 13°C (24°F) above the stamped temperature located on the thermostat.
    2.8. Do not attempt to repair the thermostat.
    2.9. If the thermostat does not function properly, replace the thermostat with a new unit that has been checked using this procedure.