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:
ES 
You can find this tutorial in Spanish here: Cómo Probar El Termostato Con Un Escáner (4.0L Ford) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.0L V6 Ford Aerostar: 1996, 1997.
- 4.0L V6 Ford Explorer: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
- 4.0L V6 Mercury Mountaineer: 1998, 1999, 2000, 2001, 2002, 2003.
BLOWN HEAD GASKET DIAGNOSTICS:
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%Disclosure: As an Amazon Associate, I earn from qualifying purchases. If my tutorials help you, using these links is an easy way to support the site at no extra cost to you. Thank you!
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: 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.
Engine Thermostat Basics
In case you're wondering why the engine has one and needs one, this section will shed some light on the subject (don't worry, I won't go into minute technical mumbo jumbo).
In a nutshell, the thermostat's job is to help regulate the engine's temperature. The PCM needs to have the engine within a certain temperature range (190° to 225° Fahrenheit) to effectively control the air/fuel mixture and emissions that the engine produces. If the engine is able to stay within this temperature range, it pollutes less and gets the best gas mileage possible.
The reason to keep the engine within this temperature range is that the cooler the engine is, the more gasoline it needs to keep running smoothly (and unfortunately, at these colder temps, it pollutes more). As the engine heats up and reaches the optimal temperature range described above, the engine needs less and less fuel to maintain it's optimal performance and of course pollutes less.
In a nutshell, here's how they both keep the engine from overheating and running optimally:
- The thermostat is the one tasked with keeping the engine from going below 190° Fahrenheit.
- The fan and fan clutch are tasked with keeping the engine below 225° Fahrenheit.
- When you start your vehicle, the thermostat is closed, thereby keeping the coolant from circulating to the radiator. This allows the engine to warm up faster.
- As the coolant circulates in the engine only, its temperature increases and when it reaches 190° F, the thermostat opens.
- With the thermostat now open, the coolant can now circulate between the radiator and the engine.
- Even tho' the thermostat has opened, the temperature of the coolant will continue to increase. This is where the fan and fan clutch come into play, since they won't allow the coolant's temperature to go beyond 227° F.
- The fan (which in this case is attached to a fan clutch) and the thermostat will work together to keep the engine's temperature between 190° and 225°F.
- If the coolant's temperature is brought down below 190° F, the thermostat will close. This helps keep the engine's temperature within the desired temperature range.
- The cool thing is that you can observe these temperature changes with your scan tool in Live Data mode and this is the way I'm gonna' show you how to test the thermostat.
Other Tips And Suggestions
Keeping on top of the cooling system, on your 4.0L equipped Ford vehicle will save you a ton of money. Yes, maintenance involves time and money, but if you let that little coolant leak go, or continue driving the vehicle even tho' it's overheating, the end result will be a major financial headache.
Overheating and coolant leak issues usually lead to blown head gaskets if they are not attended to immediately.
The following are personal recommendations:
- On late model 4.0L engines, the intake manifold gaskets are very prone to breaking and leaking antifreeze. Although this issue does not usually lead to overheating problems, it doesn't let the cooling system pressurize like it's designed to, this lead to internal hot spots in the engine that eventually lead to blown head gaskets.
- So, any time you pop the hood on your car, always check the ends of the intake manifold to see if they are coolant.
- If the radiator plastic tanks crack, replace the radiator or fix it as soon as possible. These cracks will also prevent the cooling system from pressurizing and over time and many many miles, you'll have a blown head gasket issue.
- Flush the anti-freeze at least once every two/three years. From personal experience, I don't buy into the hype that anti-freeze can last 100,000 miles and I suggest you don't either.
- If you don't or can't flush the anti-freeze, at least do a radiator drain and fill.
- Replace the thermostat as a maintenance issue, just like you would replace a fuel filter. You don't have to replace it every year, but if you plan to keep your car for a while, replace it every two years.
- Replace the radiator upper and lower hoses if they are bulging at the hose clamps or the rubber the hose is made of is too spongy.
- If your vehicle overheats while you're driving, then pull over into a safe spot and have it towed home or to your trusted automotive repair shop.
More 4.0L V6 Ford Explorer And Mercury Mountaineer Diagnostic Tutorials
I've written quite a few 'how to test' tutorials that may help you troubleshoot your Ford Explorer or Mercury Mountaineer. You can find the complete list in this index:
Here's a sample of the tutorials you'll find:
- How To Test For A Blown Head Gasket (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Test The Fuel Injectors (1991-1996 4.0L V6 Ford Explorer And Aerostar).
- How To Test Engine Compression (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Test The Ignition Coil Pack (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!
