How To Test The Thermostat (Ford 4.6L, 5.4L)

TEST 1: Thermostat Operation

How To Test The Thermostat (Ford 4.6L, 5.4L)

OK, in this part is the actual thermostat test. Before you start, I want to remind you of two very important things:

1.)  Start your troubleshooting with a completely cold engine. This is important, not only for safety reasons, but for the sake of accuracy.

2.)  Don't open the radiator cap for any reason once the engine starts to warm up.

OK, this is what you need to do:

  1. 1

    Check the coolant level in the radiator.

    The radiator (Cooling System) has to be full to complete the test. Once you've checked and/or topped of the radiator with coolant (or water), re-install the radiator cap.

  2. 2

    Check the temperature of the upper radiator hose with your hand.

    If the engine is cold (at ambient temperature), this hose will be cold. If the hose is not cold, then you'll need to let the engine cool down further.

  1. 3

    Next, check the temperature of the coolant.

    This has to be done with your scan tool in Live Data mode (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

    On most scan tools this PID will simply say: Coolant °F or Coolant °C (see the photo in the image viewer above).

  2. 4

    Start the engine and let it warm up as you observe the temperature of the coolant on your scan tool.

    If you started out with an engine at ambient temperature, your scan tool should report a coolant temperature that should be within 10 degrees of ambient temperature. For example, If it's 95° F outside, then the coolant temperature sensor should report a temp reading between 95 to 105° F.

  3. 5

    As the engine starts to warm up, the coolant temperature registering on your scan tool will eventually read 150° Fahrenheit (65° C).

    When it does, check the temperature of the upper radiator hose by hand one more time by lightly touching it.

    What you're looking for is to make sure that the upper radiator hose is still cold. In other words, it should be the same temperature you noticed in Step 2, if so, continue to step 6.

    If the hose is hot, this confirms beyond a shadow of a doubt, that the thermostat is either missing or stuck open. You can stop your testing here and replace the thermostat.

  4. 6

    Continue observing the coolant temperature your scan tool is reading (as the engine continues to run and gain more temperature). When the coolant temperature reaches 190° Fahrenheit (87° C), lightly touch the upper radiator hose once again.

    At this point, 190° F (87° C), the upper radiator hose should be hot to the touch.

    If the upper radiator hose still feels cold at this point, don't worry about it just yet, continue to the next step.

  5. 7

    Let the engine run some more.

    The coolant will eventually reach a temperature of 200° F (93° C). When it does, lightly touch the upper radiator hose once again.

    The hose should now be hot -no ifs, ands, or buts.

  6. 8

    OK, we're done. Turn off the engine and let's interpret your results below:

Interpreting The Results

CASE 1: The radiator hose got hot at 190° F and stayed hot at 200° F. This confirms that the thermostat is functioning correctly.

Here's why: This is the normal and expected result of a properly operating thermostat since it confirms that the thermostat was closed below 190° F and that it did indeed open after 190° F to let the now hot coolant circulate to the radiator.

CASE 2: The radiator hose DID NOT get hot at 190° F nor at 200° F. This confirms that the thermostat is bad and is stuck closed. Replace the thermostat.

CASE 3: The radiator hose got hot below 150° F. This confirms that the thermostat is bad and is stuck open. Replace the thermostat.

Here's why: If the thermostat were working correctly, it would not let any coolant circulate within the hose and the hose would be at ambient temperature. Since the thermostat is stuck open (or missing) the coolant starts to circulate immediately and the hose will feel warm to hot as soon as the engine has been running for a few minutes.



Ford Vehicles:

  • Crown Victoria
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • E150, E250, E350
    • 1997, 1998, 1999, 2000, 2001, 2002
  • Expedition
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009

Ford Vehicles:

  • Explorer (4.6L)
    • 2002, 2003, 2004, 2005
  • F150, F250
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008
  • Mustang (GT & Cobra)
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

Ford Vehicles:

  • Thunderbird
    • 1995, 1996, 1997

Lincoln Vehicles:

  • Aviator
    • 2003, 2004, 2005
  • Mark VIII
    • 1995, 1996, 1997, 1998

Lincoln Vehicles:

  • Navigator
    • 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009
  • Town Car
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

Mercury Vehicles:

  • Cougar
    • 1995, 1996, 1997
  • Grand Marquis
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Mountaineer (4.6L)
    • 2002, 2003, 2004, 2005