The temperature of the engine coolant is controlled by two blocking-type thermostats located in a housing attached to the right side of the cylinder head. See Figure 24251 .
1. Vent Line Connection
6. Thermostat Housing
2. Bolts, Thermostat Housing-to-Cylinder Head (4)
7. Drain Cock
3. Hose Clamps (2)
8. Seals, Thermostat Housing (2)
4. Hose, Coolant
9. Thermostats (2)
5. Bypass Tube
10. Cylinder Head
Thermostat and Related Parts
In addition to a rubber seal that is part of the thermostat, there is a lip-type seal for each thermostat that is installed in a bore in the thermostat housing. See Figure 20514 .
1. Connection Opening, Vent Line
4. Thermostat Housing
2. Seal, Thermostat (2)
5. Seals, Thermostat Housing (2)
3. Thermostat (2)
At coolant temperatures below approximately 86-89C (186-193F), the thermostat valves remain closed and block the flow of coolant from the engine to the radiator. During this period, all of the coolant in the system is recirculated through the engine and is directed back to the suction side of the water pump via a bypass tube. As the coolant temperature rises above 86-89C (186-193F) the thermostat valves start to open, restricting the bypass system, and allowing a portion of the coolant to circulate through the radiator. When the coolant temperature reaches approximately 97C (207F) the thermostat valves are fully open, the bypass system is blocked off, and the coolant is directed through the radiator. See Figure 21519 .
Coolant Flow Schematic
Properly operating thermostats are essential for efficient operation of the engine. If the engine operating temperature deviates from the normal range of 86-99C (186-211F) remove, inspect and replace the thermostats if necessary.
To determine if repair or replacement of the thermostat is necessary, perform the following procedure. See Figure 21998 .
Flowchart for Repair or Replacement of Thermostat
Drain the thermostat as follows:
To avoid scalding from the expulsion of hot coolant, never remove the cooling system pressure cap while the engine is at operating temperature. Wear adequate protective clothing (face shield, rubber gloves, apron, and boots). Remove the cap slowly to relieve pressure.
Remove the thermostat as follows:
Inspect thermostat as follows:
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.
Check the operation of a thermostat as follows:
Ensure thermostat is operational. If thermostat motion becomes impaired, overheating may result. An engine which has overheated may also cause the thermostats to become inoperative. A thermostat stuck in the open position may not allow the engine to reach normal operating temperature. The incomplete combustion of fuel due to cold operation will result in build-up of carbon deposits on the pistons, rings and valves. A thermostat that does not fully open may cause engine overheating.
Checking Thermostat Operation
To avoid injury from scalding, use lifting tools and wear heat-resistant gloves when retrieving the thermostat from boiling water.
Install new thermostat as follows:
Positioning the Seal onto Seal Installer
Thermostat Housing Seal Insertion
Thermostat Housing Seal Installation