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

In this article, I'll show you how to test the thermostat in your Ford 4.6L or 5.4L pick up (van or SUV).

As you might already be aware, a BAD thermostat will cause one of two different conditions, either it will cause your Ford pick up (van or SUV) to overheat or cause the engine to never reach normal operating temperature.

Now, you would think that the cooler the engine runs the better, but this is not the case. If the thermostat fails and stays stuck open, the heater won't work in your vehicle (if it's winter time) among several things.

To help you navigate this article, here are its main points:

  1. Important Suggestions And Tips
  2. Symptoms Of A BAD Thermostat
  3. TEST 1: Thermostat Operation
  4. Thermostat Basics
  5. Other Tips And Suggestions
  6. More Test Articles

Important Suggestions And Tips

TIP 1: To take advantage of the thermostat test in this article, you need to monitor the coolant temperature with a scan tool in Live Data mode. 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: Do not open the radiator cap under any circumstances once the engine has warmed up or is hot, whether it's running or not.

Opening the radiator cap with a hot engine and getting sprayed by the hot coolant (which is under pressure) can cause severe burns. Your safety is your responsibility... be careful, use common sense and think safety all of the time.

TIP 3: The most important factor, to successfully test the thermostat, is to start your troubleshooting with a completely cold engine.

What I mean is that the engine has to be at ambient temperature. This will guarantee the accuracy of your test. If the engine in your vehicle is hot or has been running for an extended amount of time... 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

Having established at the beginning of the article that the engine will overheat or under cool with a BAD thermostat, I'll go into more specifics about this:

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 (smog check).

You're probably surprised to find out that a missing or stuck open thermostat causes higher fuel consumption.

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, it will max out.
  4. If your vehicle is equipped with a temperature light, it will shine nice and bright to let you know that an engine meltdown is occurring.

OK, let's get testing...