Thermostat: Testing and Inspection
ThermostatA new thermostat should be installed only after the following electrical and mechanical tests have been carried out.
Thermostat - Electrical Test, 4.0L SOHC
NOTE: The electrical thermostat test is most accurate if carried out indoors at less than 37.8 degrees C (100 degrees F) ambient air. This test may be carried out with or without the hood open and with the engine warm or cold.
1. Check the engine coolant level. Fill as needed.
2. With the ignition OFF, voltage values (0 - 5 Volts) may now be monitored while the sensor retains its connection to the wiring harness.
A diagnostic tool may be used to monitor the ECT on vehicles equipped with data link connector (DLC).
3. Place the transmission in PARK (P) or NEUTRAL (N).
4. Start the engine and allow the engine to idle throughout this test. Allow the engine to run for 2 minutes, then record the ECT voltage. Record the ECT voltage every 60 seconds. When the ECT voltage trend changes direction or only changes slightly (0.03 volt or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart listed below.
5. If the thermostat opening voltage is greater than 0.58 volt and less than 90 degrees C (194 degrees F), install a new water thermostat.
6. If the thermostat opening voltage is less than 0.58 volt and greater than 90 degrees C (194 degrees F), the thermostat is good and a new thermostat should not be installed. GO to Symptom Chart for further instructions. Symptom Related Diagnostic Procedures
Thermostat - Mechanical Test
1. Remove the thermostat.
2. Check the thermostat for seating. Hold the thermostat tip to a lighted background. Leakage of light around the thermostat valve at room temperature indicates that a new thermostat should be installed. Some thermostats have a small leakage notch at one location on the perimeter of the thermostat valve, which is considered normal.
3. Immerse the thermostat in a boiling antifreeze and water mixture.
4. See the General Specifications chart for thermostat opening temperatures.