Thermostat
Thermostat - CoolantA new coolant thermostat should be installed only after the following electrical and mechanical tests have been carried out.
Thermostat - Electrical Test - 5.4L, 6.8L
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. A scan tool may be used to monitor the CHT 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.
4. Start the engine and allow the engine 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 shown.
5. If the thermostat opening temperature is less than 88°C (190°F) on 5.4L engines or 82°C (180°F) on 6.8L engines, install a new coolant thermostat.
6. If the thermostat opening temperature is greater than 82°C (180°F), the coolant thermostat is good, a new thermostat should not be installed. GO to Symptom Chart for further instructions.
Thermostat - Mechanical Test
1. Remove the coolant thermostat.
2. Check the coolant thermostat for seating. Hold the coolant thermostat up to a lighted background. Leakage of light around the thermostat valve at room temperature indicates that the coolant thermostat should be replaced. Some coolant thermostats have a small leakage notch at one location on the perimeter of the thermostat valve, which is considered normal.
3. Immerse the coolant thermostat in a boiling antifreeze and water mixture.
4. See the General Specifications chart for coolant thermostat opening temperatures.