Thermostat Diagnosis (Overheating)

Step	Action	Yes	No
IMPORTANT: The J 24731 Tempil Stick is a pencil-like device that has a wax material containing certain chemicals which melt at a given temperature. See Special Tools . Use the temperature sticks to determine a thermostats operating temperature by rubbing 87°C (188°F) and 97°C (206°F) sticks on the coolant outlet. 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. Refer to Cooling System Draining and Filling (LS4 GE 47716 Fill) or Cooling System Draining and Filling (LS4 Static Fill) or Cooling System Draining and Filling (L26 GE 47716 Fill) or Cooling System Draining and Filling (L26 Static Fill) . Rub a 97°C (206°F) J 24731 on the coolant outlet. See Special Tools . Warm up the engine at fast idle. Check for coolant flow before the mark begins to melt. Is there coolant flow before the mark melts?	Go to Step 2	Go to Step 3
2	The problem is caused from something other than the thermostat. Refer to Engine Overheating . Does the engine still overheat?	-	System OK
3	Replace the thermostat. Refer to Engine Coolant Thermostat Replacement (LS4) or Engine Coolant Thermostat Replacement (L26) . Check the system Is the engine still overheating?	-	System OK