and corrosion from the engine coolant so that it remains closed, or only partially open, thereby restricting the
flow of water, overheating of the engine will result. A thermostat which is stuck in a wide open position may not
permit the engine to reach its normal operating temperature. The incomplete combustion of fuel due to cold
operation will result in a buildup of carbon deposits on the pistons, rings, and valves.
The operation of the thermostat may be checked by immersing it in a container of hot water (Fig. 2). Place a
thermometer in the container, but do not allow it to touch the bottom. Agitate the water to maintain an even
temperature throughout the container. As the water is heated, the thermostat valve should begin to open when
the temperature reaches 167-172 (75-78 ). The opening temperature is usually stamped on the thermostat.
The thermostat should be fully open at approximately 190-192 (88-89 ).
Clean the thermostat seating surface in the thermostat housing and base or the water outlet elbow.
Check the bleed hole in the thermostat housing to be sure it is open.
Drill a 3/32-inch diameter hole in the thermostat housing used on in-line industrial engines built prior to serial
number 4D-094 (refer to Fig. 3). This will provide a coolant drain hole for the bypass cavity in the housing.
Refer to Fig. 1 and install the thermostat as follows:
1. Place a new gasket on the thermostat housing.
2. Insert the thermostat into the housing.
3. Install the water outlet elbow and secure it to the housing with two bolts and lockwashers.
4. Connect the hose from the radiator or heat exchanger to the water outlet elbow, align and tighten the
After the thermostat has been installed, close all of the draincocks and fill the cooling system. Vent the system
as outlined in Cooling System. Then start the engine and check for leaks.
Figure 2. Checking Thermostat Operation
Figure 3. Cross-Section of Thermostat Housing