GM Prizm/Nova 1985-1993 Repair Guide




All Models

See Figures 1 and 2

The thermostat and its built-in by-pass valve is installed on the inlet side of the water pump. Its purpose is to prevent overheating of the coolant by controlling the flow into the engine from the radiator. During warm up, the thermostat remains closed so that the coolant within the engine heats quickly and aids the warming up process.

As the coolant temperature increases, the thermostat gradually opens, allowing a supply of lower temperature coolant (from the radiator) to enter the water pump and circulate through the engine.

A thermostat with an internal by-pass should never be removed as a countermeasure to overheating. Removing the thermostat actually makes the problem worse because more coolant bypasses the radiator, thereby reducing cooling even more.

  1. Drain the cooling system and save the coolant for reuse.

When draining the coolant, keep in mind that cats and dogs are attracted by the 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 water inlet housing and remove the thermostat. Carefully observe the positioning of the thermostat within the housing.

Click image to see an enlarged view

Fig. Fig. 1: Thermostat assembly. Prizm shown, Nova similar

  1. Install the new thermostat in the housing, making sure it is in correctly. It is possible to install it backwards. Additionally, make certain that the air bleed valve aligns with the protrusion on the water inlet housing. Failure to observe this placement can result in poor air bleeding and possible overheating.

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Fig. Fig. 2: Correct placement of the air bleed valve during thermostat replacement

  1. Install the water inlet housing cover with a new gasket. Install the 2 hold down bolts and tighten them to 20 ft. lbs. Do not overtighten these bolts!
  3. Refill the cooling system with coolant.
  5. Start the engine. During the warm up period, observe the temperature gauge for normal behavior. Also during this period, check the water inlet housing area for any sign of leakage. Remember to check for leaks under both cold and hot conditions.