When the thermostat goes bad on your Ford 1.9L or 2.0L vehicle, it will either cause the car to overheat or, if it's winter time, the heater won't work.
In this article, I'll help you find out if the problem is due to the thermostat (either staying stuck closed or staying stuck open) without having to replace to see if it's good or bad.
I've also included the test steps you'll need to see if the fan motor is coming on at the correct temperature.
Contents of this tutorial:
Important Suggestions And Tips
TIP 1: The way I'm gonna' show you how to test the operation of the thermostat is by monitoring the temperature of the coolant with a scan tool (in Live Data mode). A simple code reader won't help, since a code reader does not have Live Data capability.
You don't need the Ford factory scan tool or an expensive professional technician level scan tool. A generic scan tool will do great. If you don't have a scan tool with live data mode? Check out my recommendation here: ZM301 OBD2 Scanner Diagnostic Tool (Amazon affiliate link).
TIP 2: Never open (remove) the radiator cap while the engine is warming up or hot.
As the engine runs and warms up, the coolant inside the radiator, becomes hot and pressurized. So, opening (removing) the radiator cap, under these conditions, can cause you severe burns. Your safety is your responsibility. Be careful and think safety all of the time.
TIP 3: Another very important thing, that is key to successfully troubleshooting the thermostat, is to start your test with a completely cold engine.
Symptoms Of A Bad Thermostat
Engine coolant thermostats are very predictable when they fail, since they generally fail in only one of two ways:
- The thermostat stays stuck open.
- It stays stuck closed.
Here are some of the specific symptoms you'll see:
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.
Most folks are surprised to find out that the cooler the engine runs, the more fuel the vehicle consumes since most folks intentionally remove the thermostat under the assumption that a cooler engine runs more efficiently.
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.
Engine Thermostat Basics
Knowing how the thermostat regulates the engine's temperature will help you to diagnose it quickly and efficiently. In this section I'm gonna' to briefly describe how the thermostat works.
In a nutshell, the thermostat's job is to help regulate the engine's temperature, so that your Ford's fuel injection computer can effectively control the emissions that the engine produces and to help you get the maximum amount of gas mileage possible.
The optimal temperature range that the thermostat (along with the radiator fan, coolant, water pump, etc.) try to keep your engine, is between 190° to 225° Fahrenheit (87° to 107° Celsius).
Why such a high temperature? Well, it's really not a high temperature since this is the engine's normal operating temperature. It all boils down to the fact that the colder the engine is, the more fuel 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 it pollutes less.
Here are some more specifics:
- The thermostat is the one tasked with keeping the engine from going below 190° Fahrenheit.
- The fan motor (or fan motors) are tasked with keeping the engine below 227° 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. When the coolant's temperature reaches 227° F, the fan or fans come on.
- The fan 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 down to around 200° F, the PCM then turns off the fan.
- 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.
- With the fan off, the coolant now begins to absorb more heat and the process I've described above repeats itself continually the whole time the engine is running.
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.
Alright, let's get testing.
TEST 1: Thermostat Operation
OK, here's the actual test but before you start, I just want to remind you of two very important things:
One, start with a completely cold engine. By cold I specifically mean that the engine should be at ambient temperature. Even a slightly warmed up engine will skew the results of your test.
Two, don't open the radiator cap once the engine has warmed up, there's simply no need to. More importantly, this is a safety precaution.
- 1
Check the level of the coolant inside the radiator (not the coolant reservoir).
Once you have topped off the radiator (if needed), re-install the radiator cap. Also, leave the hood open for the length of the entire test. - 2
Touch the upper radiator hose with your hand to check its temperature.
The hose should be cold (ambient temperature) to the touch.
NOTE: If the hose feels warm to hot, the engine has not cooled enough to accurately perform the test. - 3
Check the temperature of the coolant with your scan tool (see the photo above).
If you haven't already, connect your scan tool to the vehicle and get to Live Data mode. Scroll down to the PID for the coolant temperature. - 4
Crank and start the engine and let it idle as you keep an eye on the coolant temperature your scan tool is registering.
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. - 5
When your scan tool indicates that the coolant temperature has reached 150° Fahrenheit (65° C), 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, this tells you that the thermostat has been removed or is stuck open and that coolant is circulating to the radiator as soon as the engine starts. You can stop at this test step and replace the thermostat. - 6
When your scan tool indicates that the coolant temperature has reached 190° F (87° C), lightly touch the upper radiator hose once again.
The hose should start to feel hot or should definitely feel hot (compared to its temperature at the beginning of the test).
If the hose feels cold, don't worry about it just yet, continue to the next step. - 7
When your scan tool reports a coolant temperature of 200° F (93° C), lightly touch the upper radiator hose again.
If the thermostat is functioning correctly (and has opened to let the coolant circulate to the radiator), the hose will be hot. - 8
Alright, you're done. 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.
Here's why: the thermostat is stopping the flow of coolant to the radiator below 190° F, and this is the reason why the hose feels cool to the touch of the hand. Once the engine starts to run and warm up, the coolant absorbs the heat and when it eventually reaches a temperature of 190° F, the thermostat (which is in constant contact with the coolant) opens up. When this happens, the hose feels hot.
Now that you've confirmed that the thermostat is opening and closing, your next test is to check the radiator fan motor operation. For this test, go to: TEST 2: Electric Fan Motor Operation.
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 prevent the coolant from passing into the upper radiator hose (and into the radiator) and the hose would be at ambient temperature. Since the thermostat is stuck open (or missing) the coolant starts to immediately pass from the engine to radiator (thru' it) and the hose will feel warm to hot as soon as the engine starts to run.
TEST 2: Electric Fan Motor Operation
If in the previous test you verified that the thermostat is indeed functioning correctly, but your vehicle still overheats, there's a good possibility that the fan motor may not be working.
If the radiator cooling fan comes on, at the indicated coolant temperature, then you know that the it's not the cause of the overheating problem your Ford vehicle is experiencing.
You can easily check to see if the radiator cooling fan is coming on at the correct temperature with your scan tool in live data mode, and in this section, I'll show you how.
This is what you need to do:
- 1
Restart the engine and let it idle.
- 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 did NOT come on at 225° F - 227° F. This confirms a malfunctioning fan motor or fan motor relay or fuse.
Testing these components is beyond the scope of this article, but at least you 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 or a clogged radiator.
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 and that your vehicle is not overheating. No further testing is needed.
Other Tips And Suggestions
Keeping on top of the cooling system, on your 1.9L, 2.0L equipped Ford vehicle 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 'how to' tutorials that may help you troubleshoot the issues on your Ford equipped with a 1.9L or 2.0L engine. You can find the complete list at: Ford 1.9L, 2.0L Index Of Articles.
Here's a sample of the Ford 1.9L, 2.0L articles you'll find in the index:
- Testing The Oxygen Sensors Ford 1.9L, 2.0L.
- How To Test Engine Compression (Ford 1.9L, 2.0L).
- How To Test The Starter Motor (Ford 1.9L, 2.0L).
- >How To Test The Coil Pack (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!
