Thermostat

Thermostat-Water
A new water 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 performing this test.

NOTE: The electrical thermostat test is most accurate if performed indoors at less than 37.8°C (100°F) ambient air. This test may be performed 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 (N).
4. 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 water thermostat.
5. If the thermostat opening temperature is greater than 82°C (180°F), the water thermostat is good and should not be replaced. Refer to the Symptom Chart for further instructions.

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 the water thermostat should be replaced. Some water thermostats have a small leakage notch at one location on the perimeter of the thermostat valve, which is considered normal.
3. Immerse the water thermostat in a boiling antifreeze and water mixture.
4. See the Specifications for water thermostat opening temperatures.