Understanding and Testing Thermostats -EricTheCarGuy

It’s seems so basic, but thermostats are a very important part of your cooling system. In fact, they are the gatekeepers of the cooling system. We don’t think much about them until something goes wrong. In this video, we talk about the basics of thermostat operation and do some testing. I also take the opportunity to do a head to head comparison between an OE thermostat and an aftermarket one. You might be surprised at the results, I was.

I will say this. If you have a cooling system issue, the first thing to check for is air in the system. In fact, I cover the basic steps in diagnosing a cooling system in this article.


If you’re having trouble with no heat or intermittent heat, check out this article.


This video is part of a series of videos I plan to do this year on the ‘basics’. Instead of step by step repair videos, these cover the basic operation of the common components you deal with in auto repair. I hope you enjoy watching them as much as I’m enjoying making them.

Here’s some useful links for you.

How To Bleed a Cooling System: http://www.youtube.com/watch?v=zUpXgAJ1gjU

Solving Overheat Conditions: http://www.youtube.com/watch?v=hUzOTnsWImI

Discussion about this video: http://www.ericthecarguy.com/kunena/18-The-EricTheCarGuy-Video-Forum/49199-understanding-and-testing-thermostats#86232

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Stay Dirty


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Author: Rafael


24 thoughts on “Understanding and Testing Thermostats -EricTheCarGuy

  1. My car is in the shop. I likely need a new rad because it may be cracked. Hopefully it's not, but I can't tell where the leak originates from. But, my mechanic asked me if I wanted to replace my thermostat. It's a beater, but I must maintain it.
    Honda wants $53 for an OME HONDA thermostat. NAPA has after mzrket thermostates for $9.
    I don't think there is anything wrong with the thermostat though it has 266k on the engine. Any thoughts?

  2. I'm confused by the first comment where you state that Vehicles don't enter closed loop until operating temperature is reached 195 to 210 degrees Fahrenheit normally modern cars achieve closed-loop fueling status after about a minute which is when oxygen sensor heaters turn off

  3. Thanks, Eric… I learned a lot. I suspect my thermostat is acting already… my Civic DX '97 has 276,313 miles to be exact.

  4. 9:56 Where did you get 99.98°C from 212°F? The conversion from °F to °C is subtract 32, divide by 1.8 (or multiply by 5°C/9°F) to arrive at 100°C exactly.

  5. Back in the day! We just removed that puppy in the summer time because no heat was needed and the engine was not a droid or half computer and you could get away with it and just put that puppy back in durring the winter.

    "My how times are changing"…….Bob Dylan

  6. Can you both have over heating and cold air pouring out of your heater vents in the cab at the same time?

    I'll be checking my thermostat tonight/weekend. On the way to work the coolant gauge would fluctuate depending on throttle (more throttle, more heat. Coasting and temp would drop, and some rough run/idle. Before shutting off.).

  7. A thermostat works using a 'WAX MOTOR'. Inside the copper cylinder,
    hiding in the middle, is a powerful THERMO-EXPANSIVE WAX. Inside the wax
    motor cylinder is also a piston. Hot water heats the copper, then the wax,
    which pushes on the piston, which pushes against the tstat's water valve
    closer spring

  8. On the little air bleed on the thermostat you said to put it toward the top is that pointed toward the firewall or the radiator. Thanks Dennis

  9. You are the guy man…had heating problem took your advice…now all good…
    Redroo from Sydney Australia…
    cheers mate:

  10. Stupid question. But if you have an overheat, even after a thermostat replacement (twice), and water pump, would the only remaining option be the radiator?

  11. I went to diagnose why my car nissan Primera w10 kept overheating. I removed the thermostat housing and found that the previous owner runned the engine without a thermostat.🤷🏾‍♂️🤦🏾‍♂️
    Also did he use the upper part from the manufacturers thermostat like a spacer to fit the cap on the housing.
    The termostat housing got a sensor with a harnes wich he short cut with a paperclip to keep the radiator fan runing continuously.
    Question: Did the previous owner removed the thermostat for a reason and do i need to worry about putting a new thermostat?

  12. so when a thermo opens and will the temp sensor also start the cooling fan? i let my eng idle and i think it went to 95deg C then the fan kicked in and the temp went down to like 55 if i recall ,now does that mean i did not have any air in my system or at least behind the thermo and everything is working good or could it be the fan alone was enough to cool it drastically like that? i am thinking no it must have been also the thermo opening up and letting the coolant system do the job,,what do you think? because this can save me time and money if i dont really need to replace the thermo but juts do a coolant replacement

  13. i am not sure but i think my thermo is on closed position and may need to be changed,,i get a check eng light that im told from diagnosis is either my gas cap or my thermo but the light is not steady and sometimes goes away but this has been happening for years now and at certain times it would be steady on many drives and sometimes would go away so does it mean the thermo might be working properly at times and not at others? or could it be i got air behind the thermo giving a wrong reading to the sensor and in that case needs to be bruped? i thought the burping happens automatically through the reservoir, or should i put the car jacked on an incline and open the reservour cap and run the engine till it all burps out? thing is i want to change the coolant as its getting old now and wondering if i should bother changing the thermo first? i figured its a hard job but might be fast and easy for my mechanic and should not cost too much for him to change the thermo and then i can do the long drain process on my own

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