Ford 2003 Mustang Workshop Manual page 1852

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 2
minutes, then record the ECT (4.6L) or CHT (3.8L) voltage. Record the ECT (4.6L) or CHT
(3.8L) voltage every 60 seconds. When the ECT (4.6L) or CHT (3.8L) voltage trend changes
direction or only changes slightly (0.03 volts or less) from the previous reading, record this as
the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart
listed below for 4.6L engines only.
Coolant Temperature °C (°F) ECT (Volts)
22 (71)
43 (109)
71 (159)
82 (180)
91 (195)
97 (206)
105 (221)
5. If the thermostat opening voltage is greater than 0.75 volts and less than 82°C (180°F), install a
new water thermostat.
6. If the thermostat opening voltage is less than 0.75 volts and greater than 82°C (180°F), the
water thermostat is good and a new water thermostat should not be installed. GO to
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 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 the water thermostat in a boiling coolant and water mixture.
4. See the General Specifications chart for water thermostat opening temperatures.
Radiator Leak Test, Removed From the Vehicle
CAUTION: Never leak test an aluminum radiator in the same water that copper/brass
radiators are tested in. Flux and caustic cleaners may be present in the cleaning tank and they
will damage aluminum radiators.
NOTE: Always install plugs in the oil cooler fittings before leak-testing or cleaning any radiator.
NOTE: Clean the radiator before leak-testing to avoid contamination of tank.
3.00
2.01
1.01
0.75
0.059
0.050
0.040
Symptom