If you've been wondering if it's possible to test the thermostat (without replacing it on a hunch), the answer is YES! The thermostat can be tested while it's still on the engine.
As you may already know, the thermostat will fail in one of two ways. It will either stay stuck open or stuck closed.
With the help of this tutorial, you'll be able to find out if it's stuck open or stuck closed.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar El Termostato Con Un Escáner (4.0L Ford) (at: autotecnico-online.com).
Important Suggestions And Tips
TIP 1: To test the thermostat, with the help of this article, you'll need to use a scan tool that has Live Data capability. This is the fastest, easiest, and safest way to check to see if the thermostat is bad or not.
You don't need the Ford factory scan tool or an expensive professional technician level scan tool. A generic scan tool with Live Data capability will do great and I've written this article with this tool in mind (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
TIP 2: Coolant gets very hot, once the engine has reached its normal operating temperature. And so, once the engine has reached this temperature threshold, you should not open the radiator cap under any circumstances (whether it's running or not).
Hot coolant can cause severe burns. Your safety is your responsibility. Be careful, use common sense and think safety all of the time.
TIP 3: So that you won't misdiagnose a bad thermostat, it's very important that you start your troubleshooting with a completely cold engine.
When I say completely cold, I mean that the engine should be at ambient temperature. If the engine in your vehicle is hot or has been running for an extended amount of time, then let it cool down completely. This is important for safety reasons and (as mentioned before) for the accuracy of the thermostat test.
Symptoms Of A Bad Thermostat
Thermostats fail in one of two very predictable ways. Either they stay stuck closed or they stay stuck open. Each condition will result in different symptoms and they are:
CASE 1: If the thermostat fails and stays stuck open, you'll see the following symptoms:
- Bad gas mileage.
- If it's winter time, the heater will not work.
- If your vehicle is equipped with a temperature gauge (and not just a temperature light), it will always show you a low reading or none at all.
- Won't pass the state emission's test.
You probably weren't aware that a stuck open or missing thermostat causes gas mileage to go down the drain but it's true.
CASE 2: If the thermostat fails and stays stuck closed, you'll see the following symptoms:
- The engine will overheat.
- The fan clutch isn't able to cool the engine down.
- If your vehicle is equipped with a temperature gauge, the needle will max out.
- If your vehicle is equipped with a temperature light, it will scream that the engine is in the process of a meltdown.
Where To Buy The Thermostat And Save
The following links will help you to save in the purchase of the thermostat and the thermostat gasket:
10%Not sure if the thermostat above sets your particular Ford? Don't worry, once you get to the site they'll make sure it fits by asking you the specifics of your vehicle. If it doesn't fit, we'll find you the right one.
TEST 1: Verifying The Thermostat's Operation
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:
- Start this test with a completely cold engine. This is important for safety and accuracy reasons.
- 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
Open the hood and check the coolant level in the radiator.
Top it off with water or coolant, if needed. Once you're done, re-install the radiator cap. - 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. - 3
Connect your scan tool and check (and make a note) of the engine coolant temperature.
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 - 4
Start the engine and let it warm up.
Keep an eye on the temperature of the coolant on the scan tool. - 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 (at 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. - 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. - 7
When the coolant temp reaches 200° F (93° C), lightly touch the upper radiator hose once again.
The hose should now be hot no ifs, ands, or buts. - 8
Turn off the engine and let's interpret your results.
Let's take a look at what your test results mean:
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.
