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. With the ignition 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 the 73 Digital Multimeter to the ECT Sensor "T" Cable. Voltage values (0-5 V) may now be monitored while the sensor retains its connection to the wiring harness.
A scan tool or the Service Bay Diagnostic System (SBDS) may be used to monitor the ECT on vehicles equipped with Data Link Connector (DLC). The SBDS sequence to use for the screen is: Toolbox-Electronic Engine Control and DCL-item.

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 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 below.
5. If the thermostat opening voltage is greater than 0.75 volt and less than 85° C (190° F) on 5.4L engines or 82° C (180° F) on 6.8L engines, 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 should not be replaced. Refer to the Symptom Chart for further instruction.

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 General Specifications chart for water thermostat opening temperatures.