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 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: 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
Thermostats fail in one of two ways, they will either stay stuck open and let coolant circulate between the engine and radiator all of the time or they'll stay stuck closed.
CASE 1: 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.
CASE 2: 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
OK, let's get testing.
1.) It's important that you start out with a cold engine.
2.) 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:
With engine completely cold (by cold I mean at ambient temperature).
Remove the radiator cap and make sure the radiator is completely topped off.
Once you've verified that the radiator is indeed full of coolant, re-install the radiator cap.
Now, 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.
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.
Continued in the next page.