Thermostat - Electrical Test - 5.4L, 6.8L

WARNING: This page is about a different car, the 2003 Ford Pickup, 2003 Ford F550 Super Duty, 2003 Ford F450 Super Duty, 2003 Ford Excursion, and 2003 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

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.

Check the engine coolant level. Fill as needed.
A scan tool may be used to monitor the CHT on vehicles equipped with data link connector (DLC).

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).
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.

Fig 1: Diagnosis And Testing Chart
Courtesy of FORD MOTOR CO.
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.
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.