A faulty thermostat can lead to overheating or slow engine warm-up and negatively impact heater performance. Begin by draining enough coolant through the
Radiator drain tap until the coolant level is below the thermostat housing, indicated by the exposure of cooling tubes when viewed through the radiator filler cap. Next, disconnect the three hoses from the thermostat and remove it for later models or unbolt the thermostat housing cover for early 1500 models. For early 1500 models, withdraw the thermostat cover enough to allow the thermostat to be removed from its seat in the cylinder head. To test the unit's serviceability, suspend the thermostat by a piece of string in a pan of heated water. Using a thermometer, check its operation against the specified opening and closing temperatures. The thermostat should be replaced if it is stuck open or closed or fails to operate at the specified temperature of 176 degrees Fahrenheit (80 degrees Celsius). For later models, measure the thermostat's opening by inserting a steel rule when cold and again after heating; the difference should be between 0.315 and 0.354 in (8.0 and 9.0 mm) from the end of the thermostat hose connecting nozzle. Allow time for the thermostat to operate, and never refit a faulty unit.