Chrysler Full-Size Trucks 1967-1988 Repair Guide




See Figures 1, 2 and 3

It is always advisable to purchase new parts before proceeding with a job. This includes a gasket thermostat and coolant. If your truck has a plastic or metal petcock in the radiator to drain the fluid, a new O-ring or gasket may be needed. It is also advisable to look at the engine to see if while you are doing a job, you may need to replace a hose or wire that is damaged. Even though it may not pertain to the current procedure, it may take only a minute to fix another problem which is about to occur.

  1. Drain the cooling system to below the level of the thermostat.

When draining the coolant, keep in mind that cats and dogs are attracted to ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. Remove the upper radiator hose from the thermostat housing. Note the positioning of the thermostat. It is important that the thermostat is correctly installed.

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Fig. Fig. 1: Using a ratchet loosen the thermostat bolts

  1. Withdraw the housing bolts, then remove the housing and the thermostat.
  3. Clean the thermostat area thoroughly by scraping old gasket material off the housing and seating areas. It is very important to remove all traces of gasket/sealant because you do not want any coolant leaks after installation.

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Fig. Fig. 2: Remove housing and thermostat

To install:
  1. Using a new gasket and sealant, position the thermostat so that its pellet end (the part with the spring) is toward the engine block. On six cylinder engines, the vent hole must be up. Refit the thermostat housing and tighten its securing bolts to 25-30 ft. lbs. (34-41 Nm).
  3. Connect the upper radiator hose.

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Fig. Fig. 3: Common thermostat positioning for 6-238 and 8-318/360

  1. Fill the cooling system with a 50/50 water to antifreeze mixture. Warm the engine then inspect the upper radiator hose and the thermostat housing for leaks.

Poor heater output and slow engine warm-up is often caused by a thermostat stuck in the open position; occasionally one sticks shut causing immediate overheating. Do not attempt to correct an overheating condition by permanently removing the thermostat. Thermostat flow restriction is designed into the system; without it, localized overheating due to turbulence may occur.


  1. Check to make sure that the thermostat valve closes tightly. If the valve does not close completely due to foreign material, carefully clean the sealing edge of the valve while being careful not to damage the sealing edge. If the valve does not close tightly after it has been cleaned, a new thermostat must be installed.
  3. Immerse the thermostat in a container of warm water so that its pellet is completely covered and does not touch the bottom or sides of the container.
  5. Heat the water and, while stirring the water continuously (to ensure uniform temperature), check the temperature with a thermometer.
  7. Continue heating the water to a temperature approximately 20°F higher than the standard thermostat opening temperature. At this point, the thermostat should be fully open. If it is not, install a new thermostat.