Why Is My Car Overheating With a New Thermostat? Fixes

The question “Why is my car overheating with a new thermostat?” has probably crossed the minds of many car owners who are experiencing overheating engines.

Why Is My Car Overheating With a New Thermostat

Your first response when your car overheats is to check and replace the faulty thermostat, and if a new thermostat does not solve the overheating problem, you need to check other parts of the cooling system.

A bad water pump, inadequate or contaminated coolant, a bad radiator, failing coolant, and a leaky head gasket can also cause the car to overheat.

Read on to learn how to troubleshoot these mechanical issues.

Why Is My Car Overheating With a New Thermostat?

Your car is overheating with a new thermostat because of broken parts in the coolant system, such as a broken head gasket, a faulty radiator fan, and a bad water pump. Other reasons your car is overheating are a broken or clogged radiator and low or dirty engine coolant.

– A Faulty New Thermostat

The overheating problem may be coming from the thermostat you just installed. You expect that a brand-new thermostat will work perfectly. However, the new part could be defective or not installed properly.

Before you check other parts, ask a competent mechanic to test the thermostat, especially if you installed it at home. If the new thermostat is functional, check other parts in the engine cooling system for defects. After installing a new thermostat, car overheating is rare but possible.

– A Faulty Water Pump

The water pump pushes the engine’s coolant through the radiator for the coolant to cool off before it flows back through the engine.

When the water pump is defective, your car does not pump out the hot coolant and replace it with a cooler fluid to cool off the engine. Hence, even if your thermostat is new and functional, your vehicle overheats because of the engine overheating.

– Low or Dirty Coolant

If the coolant is sufficient and working properly, it transfers heat from the engine to protect it from freezing and overheating. The coolant consists of water and an antifreeze component. The engine coolant is only effective if it is clean and at an adequate level.

Reasons Why Is My Car Overheating With a New Thermostat

Contaminated coolant does not flow easily through the engine. The car temperature increases if the level of the coolant is low. The coolant also loses its effectiveness as it gets old. You need to change it frequently before your car starts to overheat.

– Defective Radiator

The radiator transfers heat from the coolant inside the engine to the outside. The heat transfer cools down the fluid, which in turn cools down the engine. The process requires a functional radiator fan and cap.

Any defects in any of these parts of the radiator will interfere with the cooling process and cause the coolant to maintain its high temperature.

– A Clogged Radiator

Your car will continue to overheat if the radiator is clogged. The radiator fan, cap, and fins may be functional. However, any debris or leak that clogs the radiator will affect its cooling function. Leaking coolant is a leading cause of a clogged radiator.

If the coolant is not topped properly, it will clog the hoses and their connections, and these clogs will lead to rust. If you do not check the coolant and fix the problem in time, the rust and clogs will lead to small holes in the radiator.

The coolant amount will decline, and the radiator will lose its optimal performance. The result is a high engine temperature that overheats your car.

– A Faulty Head Gasket

The head gasket separates the cylinder head and the engine block. It prevents engine oil and coolant from flowing into the combustion chamber while maintaining maximum compression. The contraction and expansion in the engine may crack the cylinder head and cause the gasket to leak. High pressure from overheating can also damage the gasket and cylinder head.

A damaged head gasket or cylinder head will allow the coolant and engine oil to leak. The leaks may increase as the pressure increases. The coolant declines to a low level that is insufficient to cool the engine.

– Bad Radiator Fan

A radiator fan blows cold air through the radiator, especially when your car is idling. The fan comes on automatically when you switch on the AC, whether the temperature of the coolant is high or low. However, it will also come on when the temperature of the coolant rises above 200 degrees.

The fan may fail if its blade or clutch breaks. You can easily diagnose a faulty fan with its speed. The normal speed is 20 seconds on a day with normal temperature and humidity. The fan should spin more than three times if you spin it when the car is off.

– Bad Radiator Cap

The engine chamber uses the radiator cap to release excess pressure. The internal pressure comes from the high engine temperature and coolant expansion. The cap valve opens when the pressure reaches its maximum level, allowing the hot coolant and excess pressure to flow into the car’s overflow reservoir.

Signs of Bad Radiator Cap

A defective cap causes your car to overheat because any cracks on the cap allow air into the cooling system. The air forms pockets that do not transfer any heat. In addition, any leaks or cracks on the cap do not allow the cooling system to build the necessary pressure to force the coolant into all parts of the engine.

– A Blocked Catalytic Converter

The catalytic converter in the exhaust pipe converts toxic emissions into less toxic pollutants. When the converter is blocked, the car may overheat as the engine works harder to power the clogged exhaust system.

The engine may overheat and then reduce its performance. If the catalytic converter is blocked, you will also notice dark smoke and a foul odor from the exhaust pipe.

– A Broken Coolant Temperature Sensor

A bad temp sensor may send the wrong message about the car temperature. The check engine light will pop up on the temperature gauge and your vehicle will start overheating.

A broken sensor can also cause the radiator fan to stop, causing the car temperature to rise. Your mechanic should check the sensor when fixing the overheating problem. 

How To Fix an Overheating Car With a New Thermostat

To fix an overheating car with a new thermostat, identify the part of the cooling system that is broken or worn out. A thorough inspection is often necessary when a car is overheating to identify all underlying causes. Your mechanic can check this for you.

Alternatively, you can also try some of the fixes mentioned below before you rush to the nearest garage.

– Check the Coolant Level and Fan Belt

It is risky to drive an overheating car. Avoid the temptation to keep driving until you get to the next gas station or mechanic’s shop. Instead, pull over and turn off the car. Give the radiator enough time to cool off, and then check the coolant amount. You may suffer severe burns if you open a hot radiator.

Fixes of Overheating Car With a New Thermostat


Top up the coolant if the level is low after the radiator cools down. Pouring cool coolant into a hot radiator may crack the radiator. It is advisable to carry pre-mixed coolant in your trunk at all times. Check if the fan belt is there and functional because a broken fan belt causes a vehicle to overheat and requires an immediate replacement.

– Turn on the Car Heater

After inspecting the coolant and fan belt, the next step is to expel the excess heat trapped in your car. To do this, turn the heater to the maximum level and turn the AC off. The heater will expel all the heat through the car vents.

– Visit a Mechanic for a Full Diagnosis

Tow or drive the car to the nearest mechanic for further diagnosis. The mechanic will repair or replace any broken parts to prevent future cases of overheating.

Professional help is advisable, given the multiple parts of the cooling system and the high risk of driving an overheating car.

Reasons My Car Overheating With a New Thermostat Conclusion

Why Is My Car Overheating Even After Replacing the Thermostat? Possible Solutions

If you recently replaced the thermostat in your car but are still experiencing overheating issues, there could be other causes and fixes for radiator hose temperatures. Consider checking the coolant level, ensuring proper coolant flow, examining the radiator for blockages, inspecting the water pump, and confirming the functioning of the radiator fans. Seeking professional help may be necessary to further diagnose and resolve the problem.


You can tell from the information shared here that replacing a bad thermostat sometimes does not solve overheating issues.

Other parts of the cooling system may require a replacement as well, so here are the main points we have covered in this guide:

  • Replacing a bad thermostat with another faulty new thermostat may cause persistent overheating.
  • Your car may overheat even with a new thermostat if the water pump, radiator, head gasket, or any radiator part is broken.
  • A low coolant level or dirty coolant causes overheating.
  • The first response when your car is overheating is to pull over, allow the radiator to cool, and then refill the coolant if the level is low.
  • Towing the car to a qualified mechanic is advisable to replace all the defective parts.

The information in this article should eliminate panic when your car starts to overheat. Remember to stay safe and follow the right procedure to avoid damaging the cooling system further.

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