Thermostat
ThermostatA new water thermostat should be installed only after the following electrical and mechanical tests have been carried out.
Thermostat-Electrical Test-5.4L (4V) Only
CAUTION: Always vent the exhaust to the outside when carrying out this test.
NOTE: The electrical thermostat test is most accurate if carried out indoors 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 key OFF, remove the Engine Coolant Temperature (ECT) sensor harness connector and attach ECT Sensor "T" Cable as a jumper between the Powertrain Control Module (PCM) and the ECT Sensor. Attach 73 Digital Multimeter to the ECT Sensor "T" Cable. Voltage values (0 to 5 Volts) can now be monitored while the sensor retains its connection to the wiring harness.
Use a scan tool to monitor the ECT sensor voltage.
3. NOTE: Running this test with the vehicle in gear or with the A/C compressor clutch engaged (running) will cause incorrect diagnosis.
Place transmission in PARK (P) or NEUTRAL (N).
4. Start the engine and allow to idle throughout this test. Allow the engine to run for two 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.
5. If the thermostat opening voltage is greater than 0.75 volt and less than 82°C (180°F), install a new water thermostat.
6. If the thermostat opening voltage is less than 0.75 volt and greater than 82°C (180°F), the water thermostat is good and a new thermostat should not be installed. For additional information, refer to the Symptom Chart. Symptom Related Diagnostic Procedures
Thermostat-Mechanical Test
1. Remove the water thermostat.
2. Check the water thermostat for seating. Hold the water thermostat up to a lighted background. Leakage of light around the thermostat valve at room temperature indicates that a new water thermostat should be installed. Some water thermostats have a small leakage notch at one location on the perimeter of the thermostat valve, which is considered normal.
3. Immerse it in a boiling antifreeze and water mixture.
4. For additional information, refer to Specifications for water thermostat opening temperature.