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
CAUTION: Always vent the exhaust to the outside when carrying out this test.
NOTE: The electrical thermostat test is most accurate if carried out at less than 37.8°C (100°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, remove the Engine Coolant Temperature (ECT) or Cylinder Head Temperature (CHT) sensor harness connector and attach ECT or CHT Sensor T Cable as a jumper between the Powertrain Control Module (PCM) and the ECT or CHT sensor. Attach the 73III Automotive Meter to the ECT or CHT Sensor T Cable. Voltage values (05 V) may now be monitored while the sensor retains its connection to the wiring harness.
An appropriate diagnostic tool may be used to monitor the ECT on vehicles equipped with Data Link Connector (DLC).
3. NOTE: Running this test with the vehicle in gear or with the A/C compressor clutch engaged (running) will cause incorrect diagnosis.
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 or CHT voltage. Record the ECT or CHT voltage every 60 seconds. When the ECT or CHT voltage trend changes direction or only changes slightly (0.03 volts or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart listed below for 4.6L (3V) engines only.
5. If the thermostat opening voltage is greater than 0.75 volts and less than 82°C (180°F), install a new thermostat.
6. If the thermostat opening voltage is less than 0.75 volts and greater than 82°C (180°F), the thermostat is good and a new thermostat should not be installed. GO to Symptom Chart for further instructions.
Thermostat Mechanical Test
1. Remove the thermostat.
2. Check the thermostat for seating. Hold the thermostat up 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 coolant and water mixture.
4. See the General Specifications chart for thermostat opening temperatures.