Thermostat Diagnosis. GMC Truck GMC K Pickup

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Pickup - 4WD 🢒 Body and Accessories 🢒 Wiring Systems 🢒 Thermostat Diagnosis

Thermostat Diagnosis Engine Overheating

Step

Action

Value(s)

Yes

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. If the coolant flows as indicated in the table, the thermostat may be worn.

Remove the radiator cap. Refer to Pressure Cap Description .
Rub a 97°C (206°F) J 24731 on the thermostat housing.
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

The problem is caused from something other than the thermostat.

Does the engine still overheat?

Go to Engine Overheating .

System OK

Replace the thermostat.

•	For gasoline engines, refer to Engine Coolant Thermostat Replacement .

•	For diesel engines, refer to Engine Coolant Thermostat Replacement .

Check the system.

Does the engine still overheat?

System OK

Thermostat Diagnosis Off-Vehicle

Thermostat Inspection and Testing

Use the following procedure in order to do an operational test of the thermostat:

Drain the coolant to a level just below the thermostat. Refer to Cooling System Draining and Filling .
Remove the coolant outlet. Refer to the appropriate procedure:

•	Radiator Hose Replacement .

•	Radiator Hose Replacement .

•	Radiator Hose Replacement .

•	Radiator Hose Replacement .

Remove the thermostat. Refer to the appropriate procedure:

•	Engine Coolant Thermostat Replacement .

•	Engine Coolant Thermostat Replacement .

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.

Thermostat Diagnosis Slow Engine Warm-up

Step	Action	Value(s)	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. If the coolant flows as indicated on the table, the thermostat may be worn.
1	Remove the radiator cap. Refer to Pressure Cap Description Pressure Cap Description. Rub a 87°C (188°F) J 24731 J 24731 on the thermostat housing. 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 3	Go to Step 2
2	The problem is caused from something other than the thermostat. Does the engine warm up properly?	--	System OK	--
3	Replace the thermostat. Refer to Engine Coolant Thermostat Replacement Thermostat Replacement (Gasoline Engines)or Engine Coolant Thermostat Replacement Thermostat Replacement (Diesel Engines). Check the system. Does the engine still fail to reach normal operating temperature?	--	--	System OK