How To Test The Thermostat (Ford 4.0L)

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 article, you'll be able to find out if it's stuck open or stuck closed.

En Español 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 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:

  1. Bad gas mileage.
  2. If it's winter time, the heater will not work.
  3. 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.
  4. 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:

  1. The engine will overheat.
  2. The fan clutch isn't able to cool the engine down.
  3. If your vehicle is equipped with a temperature gauge, the needle will max out.
  4. If your vehicle is equipped with a temperature light, it will scream that the engine is in the process of a meltdown.

OK, let's get testing.

Ford Vehicles:

  • Aerostar 4.0L
    • 1995, 1996, 1997
  • Explorer 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003
  • Ranger 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003