In this tutorial, I'll show you how to test the thermostat in your Ford 4.6L or 5.4L Ford E-Series van.
As you might already be aware, a bad thermostat will cause one of two different problems, either it'll cause your Ford van 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 isn't 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.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 4.6L, 5.4L V8 Ford E150: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014.
- 4.6L V8 Ford E250: 1997, 1998, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014.
- 5.4L V8 Ford E250: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014.
- 5.4L V8 Ford E350: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014.
F-Series Pickups: You can find the thermostat test for 4.6L and 5.4L V8 F-Series pickups here:
- How To Test The Thermostat (1997-2010 4.6L, 5.4L Ford F150, F250, F350) (at: easyautodiagnostics.com).
Crown Vic And Grand Marquis: You can find the thermostat test for 4.6L Crown Vic and Grand Marquis here:
NOTE: This tutorial applies to several other Ford, Mercury, and Lincoln vehicles equipped with a 4.6L or 5.4L V8 engine. You can see the application list in the 'Applies To' box on the left if you're using a desktop or at the bottom of this page if you're using a hand-held mobile device.
Important Suggestions And Tips
TIP 1: To take advantage of the thermostat test in this tutorial, 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 tutorial 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 tutorial 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:
- 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 (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:
- The engine will overheat.
- The fan clutch isn't able to cool the engine down.
- If your vehicle is equipped with a temperature gauge, it will max out.
- 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.
Checking The Thermostat Operation
OK, in this part is the actual thermostat test. Before you start, I want to remind you of two very important things:
- Start your troubleshooting with a completely cold engine. This is important, not only for safety reasons, but for the sake of the accuracy of your test result.
- Don't open the coolant expansion tank for any reason once the engine starts to warm up.
OK, this is what you need to do:
- 1
Check the coolant level in the coolant expansion tank.
Top off the coolant expansion tank with water or coolant if necessary. - 2
Connect your scan tool and open its Live Data feature.
- 3
Check the temperature of the upper radiator hose with your hand.
NOTE: If the engine is cold (at ambient temperature), this hose will be cold. If the hose isn't cold, then you'll need to let the engine cool down further. - 4
Check the temperature of the coolant on your scan tool.
On most scan tools this PID will simply say: Coolant °F or Coolant °C (see the photo in the image viewer above).
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. - 5
Start the engine and let it warm up.
- 6
At 150° Fahrenheit (65° C) on the scan tool, check the temperature of the upper radiator hose by lightly touching it with your hand.
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. - 7
Let the engine continue running.
- 8
At 190° Fahrenheit (87° C) on your scan tool, lightly touch the upper radiator hose once again.
At this temperature point, 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. - 9
At 200° F (93° C) on your scan tool, lightly touch the upper radiator hose once again.
The hose should now be hot -no ifs, ands, or buts.
Let's take a look at what your test results mean:
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.
