Thermostat Diagnosis

WARNING: This page is about a different car, the 2002 Oldsmobile Bravada, 2002 GMC Envoy XL, 2002 GMC Envoy, and 2002 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.

Step	Action	Yes	No
IMPORTANT: The temperature stick is a pencil-like device that has a wax material containing certain chemicals which melt at a given temperature. Use the temperature sticks to determine a thermostat's operating temperature by rubbing 87°C (188°F) and 97°C (206°F) sticks on the thermostat housing. The marks made by the sticks should melt when coolant temperatures reach 87°C (188°F) and 97°C (206°F), respectively. These temperatures are the normal operating range of the thermostat.
1	Remove the radiator cap. Rub a 87°C (188°F) and a 97°C (206°F) J 24731 on the engine water outlet housing which connects to the radiator inlet hose. See Special Tools and Equipment . Warm up the engine at fast idle. Check for coolant flow before engine reaches proper operating temperature. Is there coolant flow before either mark begins to melt?	Go to Step 2	Go to Step 3
2	The thermostat may be stuck open, prematurely opening or missing. Replace the thermostat. Refer to Thermostat Replacement (4.2L Engine) . Recheck the thermostat opening temperature as in Step 1. Does the engine still fail to reach proper operating temperature?	Go to Engine Fails To Reach Normal Operating Temperature .	System OK
3	Does the coolant begin to flow by the time one or both of the temperature stick marks on the engine water outlet housing begin to melt?	System OK	Go to Step 4
4	The thermostat may be stuck closed or opening too slowly. Replace the thermostat. Refer to Thermostat Replacement (4.2L Engine) . Recheck the thermostat opening temperature as in Step 1. Does the engine still overheat?	Go to Engine Overheating .	System OK