Thermostat Test and Diagnosis

When investigating a concern of overheating or an insufficient heater/defroster performance, perform the following:

1. Check and service any leaks:
   • Radiator pressure cap
   • Heater hoses
   • Radiator hoses
   • Water outlet connection gasket
   • Head gasket
   • Water pump gasket

2. Check the cold engine coolant level in the radiator.
   1. If the level is below the radiator filler neck, fill to the filler neck with approved coolant. Add coolant to the recovery reservoir to the FULL COLD mark.
   2. If the cold coolant level in the radiator coolant recovery reservoir is sufficient, run the vehicle for 10 minutes with the radiator pressure cap on and check for coolant circulation through the heater and radiator hoses. If the coolant drops below the FULL HOT mark, add coolant to the filler neck. Repeat the above procedure until all the air is purged from the cooling system.

      When this procedure has been completed, feel the heater inlet and outlet hoses and the underside of the upper radiator hose. The thermostat should be removed only if these hoses are cold after 10 minutes of operation.