TEST 1: Verifying The Thermostat's Operation

How To Test The Thermostat (Ford 4.0L)

The very first we need to do, to find out if the thermostat is fried or not, is to see if it opens at the correct temperature.

The temperature, at which the thermostat in your Ford 4.0L equipped vehicle should open up, is 195° F (91° C). We're gonna' monitor this temperature with the Scan Tool in Live Data mode.

Just a friendly reminder:

1.)  Start this test with a completely cold engine. This is important for safety and accuracy reasons.

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

    Open the hood and check the coolant level in the radiator.

    The radiator (cooling system) has to be full to complete the test. You can top it off with water or coolant, your choice. Once you're done, re-install the radiator cap.

    Also, leave the hood open for the remainder of the test.

  1. 2

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

    The hose should be cold (if the engine is completely cold). If the hose is not cold, then you'll need to let the engine cool down further.

  2. 3

    Check and make a note of the temperature of the coolant with your scan tool.

    On most scan tools this PID will simply say: Coolant °F or Coolant °C, like in the photo in the image viewer above (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

    If you started out with a cold engine, then 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. 4

    Start the engine and let it warm up.

    Your job now is to observe the temperature of the coolant on the scan tool.

  4. 5

    When the coolant temperature reaches 150° Fahrenheit (65° C), check the temperature of the upper radiator hose with your hand.

    The upper radiator hose should still be 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 tells you that the thermostat is either missing or stuck open. You can stop your testing here and replace the thermostat.

  5. 6

    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.

  6. 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.

  7. 8

    OK, were done... turn off the engine and let's interpret your results below:

Interpreting Your Results

CASE 1: The upper radiator hose got hot at 190° F and stayed hot at 200° F, this tells you that the thermostat is operating correctly.

Here's why: The thermostat is designed to open up at 195° F and let coolant pass from the engine into the radiator via the upper radiator hose. So, if the upper radiator hose did indeed get hot at 195°, this is a direct result of the hot coolant now passing thru' on its way 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 and is the reason why your vehicle is overheating. 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: coolant should only pass to the radiator (via the upper radiator hose) once the thermostat has opened at 190 to 195° F. So, if the Hose is getting hot before then, this is a clear indication that the thermostat is either missing or stuck open and letting coolant circulate at all times.