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How to Tell if the Thermostat is Bad: Common Symptoms and How to Fix It!

How To Replace Your Car Thermostat Housing

The thermostat in a car is the main component of the engine cooling system. The cooling system is the one responsible for keeping the engine from overheating. It is filled with liquid coolant and the thermostat regulates the flow of this coolant through the engine. The constant flow of the coolant is what keeps the engine of your car function at its optimal temperature. Since it is an important component of the cooling system, it will be a great help if you know how to tell if the thermostat is bad. This way, you can keep your engine from overheating or from severe issues. You will know if it is bad when you experience symptoms such as irregular temperature changes, very high temperature gauge reading, coolant leaks, cold engine, or engine overheating.  

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How to Tell if the Thermostat is Bad: How Do I Know if my Car Thermostat Needs to be Replaced?


Before learning how to tell if the thermostat is bad, you need to understand first how the thermostat works. Like mentioned earlier, a car thermostat regulates the coolant flow through the engine. It is the main component of the engine cooling system. 


The cooling system is filled with coolant and is connected into a loop with a radiator. This liquid coolant flows through the engine as it absorbs the heat and into the radiator’s top. The coolant will then be cooled down as it flows down the radiator by the air passing through it. 


When you first start your car, the engine is still cold and the thermostat will be closed. This means that there will be no flow through the radiator. Only the smaller bypass valve is open and the liquid coolant will only flow within the engine and through the heating system of the car. 


When the engine is running, reaching a certain operating temperature, a sensor inside the thermostat will allow it to gradually open and let the coolant flow through the radiator. This decreases the temperature and will be recirculated through the engine. During winter or in colder temperatures, when the temperature of the engine drops close to its minimum operating range, the thermostat will close again. The constant flow of the liquid coolant, with the help of the other parts of the cooling system, keeps the engine of the car to operate at its optimal temperature.     


It is vital that the thermostat opens and closes at the correct time in order to maintain the appropriate engine temperature. But just like any other car components, thermostats can become faulty over time too. It can be stuck closed or opened. If it is stuck closed, the coolant won’t be circulated through the radiator and back through the engine which can lead to very high engine temperatures. If it is stuck open, the opposite will happen, the coolant will flow constantly, causing the engine temperature of the engine not reaching an optimum level of heat. This can result in a lot of problems including engine performance issues. 


Problems with or related to the thermostat and the engine cooling system need to be prevented to ensure that your engine is always operating at its optimum temperature. Learning how to tell if the thermostat is bad is one way to prevent it since you will know if it is due for a repair or replacement. 


To know how to tell if the thermostat is bad, you need to look out for any of these symptoms.

  • Irregular temperature changes


When you start your car, like first thing in the morning, you will usually notice its temperature gauge in the cold part. As you start driving and the engine is running, you will normally see the gauge increase gradually until it reaches its midway section since it is the ideal temperature for the engine operation. 


Just by looking at the temperature gauge, you can know how to tell if the thermostat is bad. If the temperature gauge reads a dramatic spike, going higher until it reaches the very hot maximum reading, it can be an indication that the thermostat is stuck in a closed position. The best way to handle this is to stop your car. Pull over and let your engine cool down. 


If the thermostat is stuck open, you may see your temperature going up, slower than usual and stopping before it goes back to its midpoint position. If this happens, try to crank up the heater and if there is no warm air from the vents, it is an indication that you have a broken thermostat. 


A fluctuating temperature can also indicate that you have an issue with your car’s thermostat. You may see your temperature go abnormally low and climb up to an unusually high level after a while. 

  • Engine overheating


Since a faulty thermostat can be stuck closed, it prevents the circulation of the coolant into the engine. If you keep driving even when the temperature is getting higher, your engine will overheat and it can lead to severe problems. Make it a habit to check your temperature gauge and pay attention to the warning signs your car is telling you. It pays to know how to tell if the thermostat is bad.

  • Coolant leaks


Another thing that can happen when a faulty thermostat is stuck in a closed position is a coolant leak. The leak usually occurs around the thermostat housing. This can happen since when the thermostat is stuck in a closed position, the flow of the liquid coolant will be blocked and it can no longer flow into the engine. This can lead the coolant to overflow from the thermostat housing.  


You can check for leaks by looking under your car. You may see a greenish or reddish liquid leaking out of your car or spot a pool or stain on the surface underneath it if you have a coolant. The moment you spot a coolant leak, you need to have it checked and repaired immediately. Ignoring it can worsen the problem, your coolant hoses will also start to leak, plus your engine can become damaged due to excessive heat. 

  • Cold engine


If a thermostat that is stuck in a closed position can result in an overheating engine, a thermostat that is stuck in an open position can cause a cold engine. If it remains open, the coolant will constantly move through the engine regardless of its temperature. It will continue to flow whether it is hot or not, causing a cold engine. Running with a cold engine can lead to some mechanical issues with the car. 


If you experience any of the symptoms listed above, it is best that you have your thermostat checked immediately. If a thermostat replacement is needed, have it done right away. This way, you can prevent potential engine damage that can cost thousands of dollars to repair. If you know how to tell if the thermostat is bad, you also need to make sure that you don’t ignore the problem when you have determined that your thermostat is indeed bad. 


How to Tell if the Thermostat is Bad: How Do You Test a Car Thermostat?


Learning how to tell if the thermostat is bad can be very helpful, more so if you also know how to test your car’s thermostats. If you know how to inspect and test your thermostat, you will be able to regularly check and maintain your cooling system. Doing so can help you detect the problem early so you can prevent your thermostat from being stuck close or open.   


Here are some of the things you should check or do to diagnose if your thermostat is faulty.

  • Leaks


Since a coolant leak is one of the symptoms of a bad thermostat. You can check for leaks, drips, or large coolant stain spots on or around the mounting surface or on the housing. If there is a leak, you may have to replace your thermostat immediately.  


If you just had your thermostat replaced, improper installation can also cause a coolant leak as well as using the wrong seals, gasket, or sealant. You can fix the problem by checking and reinstalling the thermostat. Make sure that you strictly follow its torque specifications and the instructions.  

  • Rust


You can check your thermostat for any signs of corrosion or rust on its surfaces. Rust on the surfaces of the thermostat is caused by a contaminated coolant, a defective pressure cap that is causing air bubbles, or if there is too much water present in the system. 


A coolant can become contaminated when you use different types of coolant at the same time. Make sure that you only use a coolant that is recommended by your car’s manufacturer. Mixed coolants with different chemistries can break down, building up debris around the thermostat. This can keep it from opening properly. 


You can test your coolant for corrosion using a digital multimeter. To start, you need to open the radiator cap. Make sure that you wait until your engine has cooled down before you open it to avoid burn injuries. Next is to start the engine and warm it up by allowing it to idle at 1500 RPMS. Then set your multimeter to DC voltage. Put the negative probe on the battery’s negative terminal and dip the positive probe into the coolant. If you get a meter reading of .4 or less, it means that the coolant is good. If it reads more than that, it is an indication that the coolant’s additives which prevent electrolysis have broken down. The coolant has to be replaced. 


To correct the rust problem, you will have to replace the thermostat. But before you install a new one, you need to flush the cooling system thoroughly and refill it with the right coolant. Consult your owner’s manual to know its recommended coolant specifications. If your pressure cap is defective, replace it too. 

  • Deposit build-ups


Another thing that you need to check are deposit build-ups. Sludge or deposit build-ups on the inside can clog the thermostat and can hinder its individual components from working properly. A contaminated coolant can also be the one causing this. When this happens, you need to flush your cooling system and replace your thermostat.  

  • Removing the thermostat


One of the ways you’ll know how to tell if the thermostat is bad is by removing it and testing it. You just need to remove the thermostat from your car and place it in a pot of water on your stove. Make sure that your thermostat does not touch the bottom of your pot. You can use nose pliers to do this. Take note of the temperature using a thermometer when the thermostat starts to open. Do not pull it out yet. You need to wait until it is fully open and take note of it too. Once fully opened, pull it out of the pot and compare your notes with the specifications of your car found in your owner’s manual. If it's not the same, you may have to replace your thermostat. 

  • Check engine light and fault codes


Modern cars have been installed with a PCM and one of its tasks is to monitor the operation of your car’s thermostat. If your thermostat becomes faulty and is stuck in an open or close position, it will trigger the check engine light to illuminate. You can check for any stored fault codes. If you see some fault codes related to a thermostat such as P0128 and P0126, it means that you are dealing with a thermostat problem. 


Knowing how to tell if the thermostat is bad can save you a lot of trouble. It can be a disaster if you continue to drive a vehicle not knowing that its thermostat has been compromised. It can cause overheating that can lead to engine failure. If you have a bad thermostat and it is already due for a replacement, you need to have it done immediately. Doing so can help you prevent damages that can cost thousands of dollars to repair. 


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