Engine coolant temperature is controlled by two blocking-type thermostats located in a housing at the top of the water pump. See Figure
1.Thermostat Housing Assembly
Thermostat and Thermostat Housing
At coolant temperatures below 815C (1785F), the thermostat valves remain closed and block coolant flow from the engine to the radiator. All of the coolant, therefore, is recirculated through the engine and directed back to the suction side of the water pump via a bypass tube.
As coolant temperature rises above 815C (1785F), the thermostat valves start to open, restricting the flow of coolant into the bypass tube, allowing coolant to circulate through the radiator.
When the coolant temperature reaches 925C (1975F), the thermostat valves open fully, and all coolant is directed through the radiator. See Figure
4.Fan Clutch Assembly
5.Water Pump Assembly
Cooling System Operation
Cooling System Operation
Properly operating thermostats are essential for efficient engine operation. If the engine operating temperature deviates from its normal operating range of 81-92 5C (178-1975F), remove and inspect the thermostats. If necessary, replace them.
Repair or Replacement of Thermostat
To determine if repair or replacement of the thermostat is necessary, perform the following procedure. See Figure
Repair or Replacement of Thermostats Flowchart
Draining and Removal of Thermostat
Drain the thermostat housing as follows:
To avoid injury from the expulsion of hot coolant, never remove the cooling system pressure cap while the engine is at operating temperature. Remove the cap slowly to relieve pressure. Wear adequate protective clothing (face shield or safety goggles, rubber gloves, apron, and boots).
Remove the thermostats from the thermostat housing.
Inspection of Thermostat
Inspect thermostats as follows:
Clean all of the parts in fuel oil.
To avoid injury from flying debris when using compressed air, wear adequate eye protection (face shield or safety goggles) and do not exceed 40 psi (276 kPa) air pressure.
Dry parts with compressed air.
If a thermostat is obviously damaged, replace it.
If the thermostat housing is cracked, replace it.
Check thermostat operation as follows:
Immerse the thermostat in a metal container filled with water.
Place a thermometer in the container without allowing it to touch the bottom or sides of the container. See Figure
Checking Operation of Thermostat
While slowly agitating the water to maintain an even temperature, apply heat to the container. Allow at least ten minutes to pass before determining if the thermostats are opening in the correct temperature range.
To avoid injury from scalding, use lifting tools and wear heat-resistant gloves when retrieving the thermostat from boiling water.
As the water is heated, the thermostat should begin to open. The normal opening temperature is stamped on the thermostat: 81 C (178 F).