Key Takeaways

  • You test the car thermostat by feeling the upper radiator hose.
  • To fix a thermostat, you must replace it.
  • It’s easier to test a car thermostat while it’s on the car.
  • When you feel the radiator hose, it should be warm.
  • Thermostats are located in different places on different engines

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Car thermostats are an integral part to any car. They regulate the flow of coolant. But how do you test a car thermostat without removing it?

You test the car thermostat by feeling the upper radiator hose once the car is warmed up. If the upper radiator hose is warm, the thermostat opening means it’s working. If the upper radiator hose is cold, the thermostat is stuck closed.

I’m a mechanic with five years experience diagnosing and repairing vehicles. I receive regular training on the most recently released automotive technology. I am ASE certified and received my degree in automotive repair. And I diagnose and repair vehicles with the latest and most up-to-date tools and software.

Table of Contents

How To Test A Car Thermostat Without Removing The Core

Thermostats are a crucial component to any cooling system in a car. They operate through the use of a wax pellet. When the wax pellet is exposed to enough heat, the wax pellet expands, allowing coolant flow to the radiator.

With the coolant being able to flow to the radiator, the coolant is cooled, and pushed back to the engine. With this cool coolant being pushed back into the engine, the engine is then cooled down. All of this action happens through the use of a thermostat.

But how do you tell if your thermostat is bad? And how do you tell if you have a faulty thermostat without actually removing the thermostat? Removing the thermostat is a pain, because coolant leaks everywhere once you do. Then you have to fill the coolant back up once it’s leaked out.

And even once you get the thermostat out, it’s almost impossible to tell if it’s operating correctly, if the thermostat isn’t in the car. To diagnose a bad thermostat, the thermostat actually needs to be in the car. The wax pellet is very difficult to test if it's not in the thermostat housing. So you’re already onto the right idea by testing your thermostat with the thermostat in the housing.

The first thing you’re going to want to do is bring your car up to operating temperature. To do so, you’re going to start the car, and keep it in idle. Your car should be up to operating temperature within 5-10 minutes of operating.

Get out of your car, and if your hood isn’t popped already, pop it. Find the location of your thermostat. It really varies depending on your car, so I can’t say for sure where exactly it’s going to be. Look around the edges of your engine and in the corners of the engine. It’s usually located near one of the four corners of the engine block.

If you are unsure of the location, Google the year, make, and model with the phrase “thermostat location” tacked on after and images of where it’s located will show up. Next, you’re going to want to locate the upper radiator hose. The upper radiator hose is usually leading right off the thermostat housing.

So if you found the thermostat housing, the hose that’s connected to the thermostat housing is the hose you want to locate.

If you can’t find the thermostat, but you see the upper radiator hose, follow the radiator hose back to the engine block. The thermostat is usually at the junction where the hose meets the engine block.

When your car’s cooling system is operating properly, the thermostat is in the closed position. This means the coolant is circulating through the engine only. No cooling is happening when the thermostat is closed. At this time, feel the radiator hose we tracked down earlier. Give the hose a squeeze. It should feel cold and hollow.

This means no coolant is flowing through it during the early stages of the car warming up. This is how the car is supposed to operate. Once the coolant collects enough heat from the engine, it exposes the thermostat to that heat. The wax pellet opens and then coolant flows to the radiator.

Next, look at the temperature gauge in the car’s dashboard. It should be in the middle by now. Once the needle is in the middle, it’s now time to test the radiator hose. To do this, make sure the temperature needle on the dash is in the middle. Now, exit your car and grab the radiator hose that we located earlier. Squeeze the radiator hose and see what you feel.

The radiator hose should feel full and warm. If the hose feels hot to the touch and feels full when you squeeze it, you can be sure the thermostat is open, and everything is working at normal temperature.

If the needle on your dashboard is rising, but the hose feels cold and empty, you may have a stuck closed thermostat. This can be an issue, since it can lead to engine overheating. When your engine overheats, you run the risk of warping your cylinder head which can lead to a repair bill in the thousands of dollars.

If your car’s thermostat is stuck closed, immediately turn off the car and start thinking about how to replace the car’s thermostat. On the other hand, your car’s thermostat can be stuck open. To check if your car’s thermostat is stuck open, check the radiator hose for flow as soon as you start your engine.

To do this, start your engine. With the coolant flowing, check the radiator hose right away. Squeeze the hose and if it feels like coolant is flowing through the hose right after you start your engine, you have a stuck open thermostat. The thermostat can only be broken in two ways.

The thermostat can be either stuck in two positions. The thermostat can either be stuck open or stuck closed. If the thermostat is stuck open, coolant will flow through the radiator hose right away, but it will be cold to the touch.

If the radiator hose is stuck closed, no coolant will flow through the hose at all. If the thermostat is operating properly, warm coolant will be flowing through the radiator house after the vehicle has been warmed up.

How To Fix A Bad Thermostat

Fixing a bad thermostat is a pretty straightforward process. And the magical solution is, you replace it. Thermostats are a relatively cheap item that won’t break the bank. When you purchase it, you replace the old with the new, and everything should be in good working order.

To replace a thermostat, you need to find the location of the thermostat housing. Once you find the thermostat housing, you can start to unbolt the housing. It’s usually just two bolts connected to the engine block. But be careful, since lots of coolant might pour out as you unbolt the thermostat housing.

Try to see how much coolant leaked out of the engine block when you remove the housing since you are going to have to replace the coolant once the job is done. Once you remove the two thermostat housing bolts, remove the housing. Once the thermostat housing is removed, this should reveal the thermostat. The thermostat looks like a flying saucer with a spring attached to it.

You should be able to remove the car thermostat with your hand, but sometimes, they can get stuck inside the housing due to corrosion. If the thermostat is stuck, take a small screwdriver and pry the thermostat out of its housing.

The device should just pop right out. Car thermostat replacement is a relatively easy job, since all you have to do now is put the new thermostat in. Just place the thermostat back into its ‘home’, and put the thermostat housing back on. Bolt up the housing and reattach any hoses that were removed in the process.

Sometimes, this may look like reinstalling hose clamps to ensure the lips of the hoses don’t slip off during engine operation. With everything re installed, clamps clamped down, and bolts tightened to spec, you can start adding coolant.

Remember before when I said to try and estimate how much coolant you lost when you removed the thermostat housing? That is where this step comes in handy. That’s because now that the job is done, it’s time to replace the coolant you lost during the repair.

So try and add the estimated amount of coolant you lost during the repair. Once you have added the correct amount of coolant, your engine should be operating normally. Start up the engine and check that everything is working well.

After you start your engine, check that there are no leaks coming from the engine. Once confirmed that there are no leaks, next, wait for the car to get up to operating temperature. You can tell the car is at operating temperature by looking at the temperature gauge on the dashboard. Once it’s midway, it’s ok to start checking hoses.

With the car at operating temperature, grab the upper radiator hose and feel if it’s warm or not. If the upper radiator hose is warm, you have completed the job successfully. If the upper radiator hose isn’t warm, something else is going on. In this case, you may want to take your car to a mechanic to figure it out.

Where Are Thermostats Located?

Thermostats on cars are located in all different sorts of spots. What I personally do is start to look at each of the four corners of the engine. Imagine the engine is a cube, and look towards each corner for the thermostat housing.

In one of these corners the thermostat is usually residing. Very seldomly does the thermostat reside in the middle of the engine block somewhere, but sometimes it does happen. But the best way to find a thermostat is to follow the upper radiator hose.

The upper radiator hose is the hose that connects the car thermostat housing to the radiator. Usually, it’s the largest hose you can see in the engine bay. The hose is usually located in front of the engine and it’s attached to the radiator.

If you follow the radiator hose from the radiator to the engine block, you will find the thermostat. The junction where the engine block and the radiator hose meet is where you will find the thermostat. So when in doubt, just follow the radiator hose from the radiator, to the engine block, and you will find the thermostat housing, and in turn, the thermostat.

How Much Does It Cost To Replace A Thermostat?

There really isn’t a standard price for replacing a thermostat. This is because depending on where the thermostat is, it makes the job hard or easy. A difficult thermostat means it's buried somewhere deep inside the engine compartment.

This thermostat could take hours to change. The average independent mechanic will charge around $60-120 an hour to change, so you could be looking at a $400 dollar bill. In this case, the thermostat was buried deep inside the engine and was a pain to remove. Other components had to be removed in order to change this thermostat.

Other thermostats are easy to change. Sometimes, they are located right on top of the engine block. When this is the case, the mechanic will only take about 15 minutes to change the thermostats. When this good fortune happens to you, you could be looking at a bill for less than $100 dollars.

So, as you can see, changing a thermostat can be an easy job or a difficult job depending on where the thermostat is located. And this has the effect of making your bill low or high, depending on where the thermostat is located.

How To Test A Car Thermostat Without Removing The Core

About The Author

Charles Redding

Charles Redding

I've spent many years selling cars, working with auto detailers, mechanics, dealership service teams, quoting and researching car insurance, modding my own cars, and much more.

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