When the thermostat fails, one of two things will happen. Either the engine overheats or stays so cool that the heater will not work.
Thankfully, there is a way to test the thermostat to find out if it has failed or not, without having to replace it first.
In this article, I'll show how to do it in a few easy steps. If your vehicle is overheating, I have also included a brief test of the fan motor, to see if it's coming on at the correct temperature (if your specific Ford is equipped with a fan motor and not a fan clutch).
Contents of this tutorial:
Important Suggestions And Tips
TIP 1: The easiest way to test the thermostat is with a scan tool that can read Live Data and this is how I'm gonna' show you how to do it in this article.
You don't need the Ford factory scan tool or an expensive professional technician level scan tool. A generic scan tool will do just fine. Now, if you don't have a scan tool and need to buy one, check out my recommendation: ZM301 OBD2 Scanner Diagnostic Tool (Amazon affiliate link)).
TIP 2: I'm gonna' ask you to start and run the engine till it warms up, to test the thermostat. Therefore you should not open the radiator cap under any circumstances.
Opening the radiator cap with a warmed up or hot engine can cause you severe burns. Your safety is your responsibility. Be careful and think safety all of the time.
TIP 3: To successfully test the thermostat, it's very important that you start your troubleshooting with a completely cold engine.
Symptoms Of A Bad Thermostat
A engine coolant thermostat will usually fail in one of two ways:
- Stay stuck open and let coolant circulate between the engine and radiator all of the time.
- Stay stuck closed and stop coolant from circulating.
If the thermostat is 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.
A lot of folks are usually surprised to find out that a stuck open or missing thermostat has such a negative impact on gas mileage but it does.
If the thermostat is stuck closed, you'll see the following symptoms:
- The engine will overheat.
- Cooling fans come on, but the engine does not cool 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.
Alright, let's get testing.
TEST 1: Thermostat Operation
Before we get started, I want to point out two very important conditions we need to meet before starting the test:
- It's important to start with a cold engine.
- Do not open the radiator cap for any reason once the engine starts to warm up.
OK, this is what you need to do to get this show on the road:
Ray, what would be a better way to reorganize the following content block, while maintaining my exact vocabulary and language as much as possible, and not adding any new stylistic flourishes, making the new version sound like me?- 1
Check the coolant level in the radiator and top it off if necessary.
IMPORTANT: Do this with a completely cold engine to avoid getting burned by hot coolant.
Once you've verified that the radiator is indeed full of coolant, re-install the radiator cap. - 2
Check the temperature of the upper radiator hose with your hand.
The hose should be cold (ambient temperature). If the upper radiator hose is hot, then you need to let the engine cool down completely and start the test then.
NOTE: Don't skip this step since you need to make sure that the upper radiator hose is cold (ambient temperature) before cranking and starting the engine. - 3
Connect your scan tool to the vehicle and get to its Live Data mode.
- 4
Scroll down to the PID (parameter ID) for the coolant temp sensor.
- 5
Leave the hood open and start the engine while keeping your eyes on the coolant temperature sensor's values on your scan tool.
If you started out with an engine at ambient temperature, your scan tool should report a coolant temperature sensor 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. - 6
When the coolant temp sensor shows 150° Fahrenheit (65° C) on your scan tool, lightly touch the upper radiator hose to check its temperature.
The hose should still be cold (ambient temperature). In other words, it should be the same temperature as before you started the engine, if so, continue to step 6.
If the hose is hot, then this confirms that the thermostat is stuck open or missing. You can stop here, since there's no further need to go on to the next steps. Replace the thermostat. - 7
When the coolant temp sensor shows 190° Fahrenheit (87° C), lightly touch the upper radiator hose once again.
You should feel a definite warming up of the upper radiator hose (compared to its temperature at the beginning of the test).
If the hose does not feel like it's heating up, don't worry about it just yet, continue to the next step. - 8
Let the engine run till the scan tool reports a temperature of 200° F (93° C) and lightly touch the upper radiator hose again.
If the thermostat is functioning correctly, the hose will be hot. - 9
Turn the engine off and let's interpret your test results.
CASE 1: The radiator hose got hot at 190° F and stayed hot at 200° F. This confirms that the thermostat is functioning correctly.
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.
TEST 2: Electric Fan Motor Operation
If you have already confirmed that the thermostat is indeed working fine, and yet your car or minivan is still overheating, this section might help.
Before we start, let me tell you that this test is only applies to Ford 3.0L, 3.8L vehicles with an electric fan motor.
One more thing, the AC has to be turned off to successfully complete this test. Alright, this is what you'll need to do:
- 1
Restart the engine in your car or minivan.
- 2
Go to your scan tool's live data mode and check the coolant temperature PID once again.
- 3
Let the engine idle till the coolant reading reaches a temp of 225° F (107° C).
225° F (107° C) is the temperature at which the radiator fan motor should start and run to cool the engine down.
IMPORTANT: If the coolant temp reaches 227° F and the fan or fans have not come on, immediately turn off the engine and let it cool down. - 4
With the fan (or fans) running, your scan tool should be reporting a decreasing coolant temperature reading.
Once the temperature goes down to about 200° F, the fan motor should turn off. - 5
Once the temperature goes down to about 200° F, the fan motor should turn off.
Let's interpret the result of your test:
CASE 1: The fan (or fans) did NOT come on at 225° F - 227° F. This confirms that the overheating problem your vehicle is experiencing is due to a fan motor issue.
The next step is to check the fan motor itself. Other components that can cause the fan motor not to operate are: a bad fan motor relay, or/and a blown fan motor fuse. Although, testing these components is beyond the scope of this article, you now at least have a solid lead on what's causing your vehicle to overheat.
CASE 2: The fan (or fans) did come on at 225° F - 227° F, but the vehicle continues to overheat. If you have also confirmed that the thermostat is good (in TEST 1), then the most likely cause of the overheating issue is a blown head gasket.
You can find three specific tests, to verify a blown head gasket in this tutorial: How To Test For A Blown Head Gasket (Ford 3.0L, 3.8L).
CASE 3: The fan (or fans) did come on at 225° F - 227° F but the temperature DID go down. This confirms that the fan(s) and thermostat are OK. No further testing is needed.
Engine Thermostat Basics
In case you're wondering why the engine has one and needs one, well this section might 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 to effectively control the emissions that the engine produces and to help you get the maximum amount of gas mileage possible. This temperature range is between 190° to 225° Fahrenheit (87° to 107° Celsius).
The reason for this is that the cooler the engine is, the more gasoline it needs to keep running smoothly (and unfortunately, at these 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:
- When you start your vehicle, the thermostat is closed, 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.
- From this point forward, the thermostat will keep the engine from going below 190° Fahrenheit.
- With the thermostat open, the coolant can now circulate between the radiator and the engine.
- Even tho' the thermostat has opened, the coolant temperature will continue to increase.
- When the coolant reaches 227° F, the fan motor (or fan motors) come on.
- From this point forward, the fan motor or fan motors are will keep the engine below 227° Fahrenheit.
- The fans will bring down the temperature of the coolant (inside the radiator), which is circulating to the engine (due to the open thermostat). Once the temperature decreases to around 200° F, the PCM turns off the fan or fans.
- If the coolant temperature drops below 190° F, the thermostat will close. This helps keep the engine's temperature within the desired range.
- With the fans off, the coolant begins to absorb more heat, and the process I’ve described above repeats itself continually the whole time the engine is running.
Other Tips And Suggestions
Keeping on top of the cooling system, on your 3.0L, 3.8L equipped Ford car, minivan or pickup will save you a ton of money. Yes, maintenance does involve spending 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:
- 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 twice a year. 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, pull over into a safe spot and have it towed home or to your trusted automotive repair shop.
More Test Articles
I've written quite a few 3.0L, 3.8L 'how to' tutorials that may help you troubleshoot the issues on your Ford car, pickup or SUV. You can find the complete list at:
Here's a sample of the Ford 3.0L, 3.8L V6 articles you'll find:
- How To Test For A Blown Head Gasket (Ford 3.0L, 3.8L).
- How To Test The Ford EGR Valve EGR Vacuum Solenoid, DPFE Sensor (at: easyautodiagnostics.com).
- How To Diagnose Misfire Codes (Ford 3.0L, 3.8L)
- How To Test The Fuel Injectors (Ford 3.0L, 3.8L)
- How To Test The Coil Packs (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!
