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The temperature of the engine coolant is controlled by a thermostat located in the water manifold. See Figure 27957 . ‪

1.Thermostat Location ‪

Thermostat Location

At coolant temperatures below approximately 86-89 C (186-193 F), 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 86-89 C (186-193 F) 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 97 C (207 F) the thermostat valves are fully open, the bypass system is blocked off, and the coolant is directed through the radiator. See Figure 27239 . ‪

Turbotronic 638 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 86-99 C (186-210 F) remove, inspect and replace the thermostats if necessary. ‪

Repair or Replacement of Thermostat

To determine if repair or replacement of the thermostat is necessary, perform the following procedure. See Figure 27227 . ‪

Flowchart for Repair or Replacement of Thermostat

Removal of Thermostat

Remove the thermostat as follows: ‪

Do not remove the pressure control cap from the radiator or attempt to drain the coolant until the engine has cooled. Once the engine has cooled, use extreme caution when removing the cap. The sudden release of pressure from a heated cooling system can result in a loss of coolant and possible personal injury (scalding, eye injury, etc.) from the hot liquid. ‪

  1. Drain the coolant system.
  2. Remove upper radiator hose.
  3. Remove four thermostat housing bolts.
  4. Remove thermostat housing cover.
  5. Remove thermostat from water manifold. Discard gasket.
Test of Thermostat

Test the operation of 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. ‪

  1. Immerse the thermostat in a metal container of water. See Figure 20516 .

    Checking Thermostat Operation

  2. Place a thermometer in the container using care not to allow the thermometer to touch the bottom or sides of the container.
  3. While slowly agitating the water to maintain an even temperature, apply heat to the container. Allow at least 10 minutes for the thermostats to react before determining if the thermostats are opening in the correct temperature range and are fully opened at 108C (227F).

    Use caution when performing this procedure. Personal injury (scalding) may result from contact with the hot liquid. ‪

  4. As the water is heated, the thermostat should begin to open (the normal opening temperature is stamped on the thermostat).
  5. If thermostat does not operate properly replace.

Installation of Thermostat

Install thermostat as follows: ‪

  1. Install thermostat into water manifold housing.
  2. Install thermostat cover to the water manifold and secure using four bolts.
  3. Install upper radiator hose.
  4. Fill cooling system.
  5. Run the engine to verify repair.