Remove the hose clamp attaching the upper coolant hose to the inlet.
For non-EGR engines remove the mounting bolts attaching the inlet to the coolant pump.
For EGR engines, remove three bolts securing the upper thermostat housing to lower thermostat housing.
Collect any coolant that runs out of the inlet or coolant pump.
Remove the thermostat(s) and O-ring(s) from the coolant pump. Discard the O-ring(s).
Check the thermostat(s) for proper operation. Refer to section
. Replace any thermostat that fails one or more tests.
Check the thermostat as follows:
On 6-cylinder engines, there are two thermostats. On 4-cylinder engines, there is only one thermostat.
Hang the thermostat by a wire inside a container filled with water. See Figure
Heat the Thermostat
To allow the thermostat to heat evenly, make sure it is not touching the sides of the container.
To prevent damage to the thermostat or heating container, do not ever use a welding torch or soldering gun as a source of heat.
Heat the water with a suitable heating element. Stir the water occasionally to help equalize the temperature.
To avoid injury from scalding, use lifting tools and wear heat-resistant gloves when retrieving the thermostat from boiling water.
Measure the water temperature. As the temperature climbs to around 75C (165F), slow the rate of heating down to about 1 to 2C (2 to 3F) per minute.
In about five minutes, the main valve should begin to open.
Continue to heat the water at this rate for about six to eight minutes until it reaches the opening temperature of the main valve. If the main valve is not completely open, replace the thermostat. See Figure
3. Short Circuit Valve
2. Main Valve
Check all the test values listed in Table
. If any of the values are not met, replace the thermostat.
Main Valve Starts To Open
81 to 85C (178 to 185F)
Short Circuit Valve Fully Closed
Main Valve Fully Open
Main Valve Minimum Lift (A)
8 mm (0.3 in.)
Thermostat Test Values
At the temperature of 85C (185F), check that the main valve has started to open.
At the temperature of 92C (198F), check that the short circuit valve is fully closed.
At the opening temperature of 95C (203F), check that the main valve is fully open.
Check the minimum lift of the main valve at the opening temperature.
Install the original or replacement thermostat(s), as necessary.
Install the upper coolant hose inlet and new O-ring(s) on the coolant pump.
Fill the engine with the removed coolant, and check for leaks and proper operation.