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Car Thermostat Replacement: 10 Important Tips

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

A bad thermostat can cause engine overheating or not allowed the engine to reach operating temperature.

A bad thermostat can cause engine overheating or not allowed the engine to reach operating temperature.

How to Change a Thermostat

Car thermostat replacement is not difficult. But, you do need to pay special attention to key points in the process.

What to Know

  • How to access the thermostat housing
  • Using the right tools
  • Preparing the surfaces for the new installation
  • Getting the correct thermostat for your application
  • Installing the new thermostat correctly
  • Bleeding the cooling system to prevent overheating

Overlooking a small detail that may seem inconsequential at first can lead to cooling system problems and more expensive repairs.

The 10 essential tips presented in this guide will help you replace your car thermostat properly. They'll help you avoid issues like coolant leaks, thermostat housing damage and stripped threads associated with poor thermostat service.

You'll find most car thermostats near the top and to one side of the engine—which makes your job easier—and less frequently toward the lower section of the block. And, while you may need to remove one or two accessories to gain access to the thermostat, you can replace it in your own garage in about an hour or so using a few simple tools.

If you know your car thermostat has failed, go ahead and change it using these tips. However, if you don't know whether the thermostat is bad, check this other article about troubleshooting the thermostat.


Some Tools You May Need

Car Thermostat Replacement

1. Cool the Engine

2. Drain Some Coolant

3. Access to the Thermostat Housing

4. Use the Appropriate Tool

5. Remove the Thermostat Housing

6. Check Thermostat Alignment

7. Clean the Mating Surfaces

8. Get a New Thermostat

9. Use the Appropriate Seal

10. Install the New Thermostat

A Systematic Approach to Car Thermostat Replacement

Some Tools You May Need

* Catch pan

* Floor jack, if necessary

* 2 jack stands, if necesary

* Ratchet and six-point socket set

* Standard and/or Phillips screwdriver

* Rags

* Plastic scraper

* Fresh coolant, if necessary

* Chocks

* Six-point wrench set

* Torque wrench

A stuck thermostat can overheat and seriously damage your car engine.

A stuck thermostat can overheat and seriously damage your car engine.

Replacement Process

The best way to go about your repair job—if this is your first time replacing a car thermostat—is to use the service manual for your particular vehicle make and model.

However, the next ten tips may be sufficient to replace the thermostat in many models since the unit is readily accessible, and you may not need to remove components, except for some parts from the air cleaner assembly.

If you still need some help, though, the manual provides a step-by-step procedure to replace the thermostat for your specific model, how to deal with components that might block access to the thermostat housing, torque specifications, and the bleeding process for your vehicle, if necessary.

Buy an inexpensive, aftermarket copy online like this Haynes manual, from Amazon. It comes with many step-by-step procedures for many maintenance, troubleshooting, and replacement tasks that will help you in the future. So the manual pays for itself in a few days.

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Following your service manual, pay attention to the next important tips to avoid potential repair problems associated with car thermostat replacement.

So, let's start replacing the thermostat.

Make sure the engine is cool before you start replacing the thermostat.

Make sure the engine is cool before you start replacing the thermostat.

1. Cool the Engine

Wait for the radiator and engine to cool before attempting to replace the thermostat. When hot, the cooling system is under pressure and you'll seriously burn yourself unintentionally by releasing a gush of hot or boiling coolant.

2. Drain Some Coolant

Drain about two quarts of coolant using the radiator drain valve before disconnecting the upper radiator hose from the thermostat shell. This will bring the coolant level below the thermostat housing and prevent coolant from splashing over belts and other components after separating the upper radiator hose.

  • Coolant causes some parts, especially rubber, to deteriorate over time. You may need to raise the front of your vehicle and remove a splash shield to reach the radiator valve. After raising your vehicle, chock the rear wheels and support your car on jack stands.

3. Access the Thermostat Housing

Disconnect the upper radiator hose from the thermostat housing. To locate the thermostat housing:

  • Follow the upper radiator hose back to the engine. On some models, you need to remove the air filter assembly to clear the area around the thermostat case. If necessary, disconnect the thin hose and electrical sensor connector from the thermostat housing.
  • After removing the clamp securing the upper hose to the thermostat casing, you may find out that the hose refuses to come off. To break the seal, carefully insert a thin, flat-head screwdriver at different points between the hose and the fitting.
  • Then, twist the hose back and forth while carefully pulling it off the thermostat housing.
Use a line wrench or six-point socket tool to prevent damage to bolts.

Use a line wrench or six-point socket tool to prevent damage to bolts.

4. Use the Appropriate Tool

Use a six-point wrench or six-point socket of the correct size when removing the two or three housing mounting bolts. Using a six-point tool will avoid rounding off the bolts' heads.

5. Remove the Thermostat Housing

When trying to lift the thermostat housing, if you notice it doesn't move, lightly tap it with a rubber mallet or screwdriver's plastic handle.

  • Do not use a wrench or a regular hammer. Thermostat casings are made of aluminum or plastic and can easily break.

6. Check Thermostat Alignment

Carefully lift the thermostat housing away from the engine. Before removing the thermostat, pay attention to the way the thermostat aligns inside the housing.

  • To check the alignment, note which direction the bleed pin—near the edge on the round base of the thermostat—points to.
  • You will have to install the new thermostat with the bleed pin pointing in the same direction as the old one.
  • Some thermostats come with a guiding tab that fits into a recess on the mounting base to prevent maligning the thermostat.

7. Clean the Mating Surfaces

After removing the thermostat, cover the thermostat opening on the engine with a clean rag to prevent sealant or gasket material from going into the engine.

To clean the thermostat housing and engine mating surfaces:

  • Use a plastic scraper, if you need to remove old gasket material and sealing compound. Avoid using a metal scraper that can gouge the mating surfaces. This may lead to coolant leaks.
Make sure the new thermostat aligns perfectly inside before tightening the mounting bolts.

Make sure the new thermostat aligns perfectly inside before tightening the mounting bolts.

8. Get a New Thermostat

When buying the new thermostat, get one with the same temperature rating your car manufacturer specifies for your vehicle. You'll find this information in your vehicle service manual.

  • Most vehicle models use a 192F or 195F rated thermostat.
  • If you install a thermostat with a different rating temperature, you'll end up with all kinds of performance problems like fuel, emission and ignition system problems, because your engine will operate out of its normal temperature range.

9. Use the Appropriate Seal

Most new thermostats come with their own gaskets or rubber seal, but not all. So check that your new thermostat includes one; otherwise, buy one.

  • Remember that you may need to apply a sealant to the new gasket—check your vehicle service manual. Only use the sealant recommended by your car manufacturer and use a small amount. Using too much sealant may damage an oxygen sensor and disrupt engine performance.
  • Some thermostats come with a rubber seal instead of a traditional gasket. Before installing, apply a light coat of fresh coolant to the seal.
  • Some models use a liquid gasket instead. Consult your repair manual, if necessary.
  • Verify that the mating surfaces on the thermostat housing and the engine block are perfectly clean.

10. Install the New Thermostat

Compare the new thermostat to the old one. Then, position the new one in place along with its seal, and tighten the housing bolts using a torque wrench.

  • Look for a marking on the thermostat indicating which end should face the radiator. If you can't find the marking, just make sure the spring (sensor) end faces the engine.
  • Also, some thermostats have locating pins that will help you to position the thermostat in the housing and block.
  • Your car repair manual specifies the torque value for the thermostat mounting screws. This will prevent cracking the plastic shell, leaving the shell loose and causing coolant leaks, or damaging the bolts' threads.
  • When ready, top up the radiator with coolant to replace the amount you removed, and bleed the cooling system, if your car manufacturer recommends it—you can follow one of the bleeding procedures described in this post about how to change coolant, or check your vehicle service manual.
  • Trapped air will cause your engine to overheat. The next video gives you an idea about how to bleed the system in your vehicle as well, using a special funnel designed for this purpose.
  • Then, test your repair job by starting the engine and checking for coolant leaks and the coolant level.

A Systematic Approach to Car Thermostat Replacement

A car thermostat replacement job is within the reach of the average car owner. You don't need special skills or tools. And you can do it in your own garage. But it doesn't mean that nothing could go wrong.

The little details will get you in trouble when you don't know or forget about them. Doing a hit-or-miss repair job or following the wrong procedure will result in a much more expensive job than taking your car to the repair shop in the first place.

If in doubt, consult the vehicle repair manual for your particular car make and model. The ten tips described here point out the most common details you are most likely to miss and get you in trouble when replacing your car thermostat. So keep these tips handy and consult them as necessary.

And here is a video that can also help you visually to complete your repair job.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: I changed my car thermostat, and now it overheats. This only happens when the engine is cool, and you drive like one or two miles. Once it overheats, I just loosen the radiator cap, and the car goes back to normal. What could be the problem?

Answer: There could be air trapped in the system. Sometimes it is necessary to purge the thermostat system more than once.

Question: I got my car thermostat replaced and had to fill the coolant three times. Is that normal?

Answer: It is possible. When you first install the thermostat, it is closed. Then you try to top off the system, but there remain some air pockets that either you need to bleed or the system itself will 'burp', like in some newer cooling systems.

Question: Do I apply the sealant for the thermostat replacement towards the motor?

Answer: Yes, make sure the gasket is properly centered. Some gaskets come with an adhesive backing that will hold the thermostat in place, as well.

Question: Once I change my car's thermostat will the raffling sound stop?

Answer: Maybe not. You need to locate the source of the sound. A piece of rubber hose may help here.

Question: What is a good tip to assure that the thermostat is always facing the right way? The coolant usually flows top to bottom of the radiator, right? So which end would I want facing the flow?

Answer: Have the spring face the engine; the pointy side towards the radiator.

Question: I have put a new thermostat and housing but now the fan kicks in after a while and never stops running. What could be the problem?

Answer: The thermostat might be sticking closed or partially open. Check the temperature gauge when the fan kicks in and see if there are any movement of the gauge after a while. This other post may help you do a little diagnostic:

Question: I changed my thermostat because my car was giving a code that said it was bad. Also, the heater stopped working but I wasn't having overheating problems. After changing it, the code still hasn't gone away and my heat still isn't working. Do you have any suggestions?

Answer: Trouble code P0128 is usually related to a faulty thermostat, but a faulty intake air temperature (IAT) sensor, faulty radiator fan, bad engine coolant temperature (ECT) sensor and low coolant level can trigger this code. Make sure the heater is not clogged or leaking.

Question: How long should I let the thermostat seal sit before I check for leaks ?

Answer: It takes a few hours. But check the instructions that come with the product to be safe.

Question: Can you replace the thermostat without noticing that the housing is leaking and needs to be replaced?

Answer: It may be possible if the leak is small and hidden from view. If this is a gasket issue, it'll be fixed when you correctly installed the new one. If the housing is cracked, the leak probably will return.

Question: Does the gasket go on first before the thermostat when replacing it?

Answer: It depends on your particular model. Whatever works for your particular configuration. Make sure the sealing surface is clean before installing the gasket. Put everything in place without the mounting bolts and make sure it's going to seal properly.

Question: What kind of coolant thermostat is best for my 1999 Pontiac Grand Am? Should I buy the 180 degrees or 190 degrees heat resistance?

Answer: If you have the 4 cylinder model, you may use the 180; for a 6 cylinder you can use the 190 degrees range. It runs a little hotter. If possible, you might want to check your vehicle service manual. If you don't have a copy, look it up in the reference section of your local library.

Question: Does a thermostat have a serial number one can use to identify a specific one for each car?

Answer: Not necessarily. One serial number may apply to more than one model. If you want to be absolutely sure you're getting the correct replacement, you can use your car's VIN number located on your registration card or the interior of the dashboard, driver side.

Question: Where should the jiggle pin face on a 351 clevland?

Answer: If you are using the special 351c thermostat, the pin points to the radiator. This other post shows also the 351C Robert Shaw thermostat.

Question: How do I get my car to stop leaking around the bottom bolt holding the thermostat? I have gotten a new one and gasket yet it is still leaking and didn't until I replaced the water pump.

Answer: Double check the mounting bolts are torqued to specs and using the proper mounting bolts. Consult the repair manual for your particular make and model. Then, make sure the O-ring or gasket was properly installed. Also, the thermostat housing or mounting surface might be damaged and not sealing properly. Sometimes you may be able to use RTV sealant if the mounting surface is damaged. Or replace the housing, if necessary.

Question: I changed my thermostat but did not use a new gasket. I reused the original rubber gasket. The check engine light is still only. The temperature gauge is not working at all. The air temp in the car does warm up, but not really hot like previously. The thermostat is not OEM - aftermarket but good reviews. I'm still getting codes P0125 & P0128. Any suggestions?

Answer: Make sure the coolant level is OK. There can also be a problem with the coolant temperature sensor (CTS) or temperature sending unit. If the sensor is running erratically, the computer may be running the cooling fan for longer than needed. Have the CTS air temperature sensor checked.

Question: I just purchased a new thermostat for my 1995 Chevy Blazer, 4.3liter. And my seal is way larger than the radius of the thermostat. It doesn't fit around the edge of the thermostat itself. Is this size difference normal for a 1995 Chevy Blazer thermostat?

Answer: You may have the wrong seal for the application. Usually, a seal will fit into a groove on the mating surface to properly block any leaks. Check closely and see if you can place the seal in its groove; otherwise, check with your local auto parts for a new seal and compare it to the mating surface of the thermostat housing before you buy it.

Question: Can I use the same rubber from my old thermostat on my new thermostat?

Answer: It's better to replace it. They also wear from the heat and eventually fail.

Question: I just purchased a new thermostat and gasket. However, the thermostat will not fit with the gasket on it. Is there a particular method of installing the thermostat that won't cause the gasket to come off?

Answer: Make sure the old gasket is not stuck to the mating surface. Clean any residue around both surfaces. Also, double-check that you got the right part for your application. You may need an OEM part if you got a different one. Another possibility is that the gasket is shifting position when you try to install it. Try holding the gasket in place by adding a thin layer of petroleum jelly.

© 2014 Dan Ferrell

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