What Causes a Car to Overheat? 12 Reasons Why!

Even with modern and powerful heating systems, a car’s engine can still overheat. This usually happens when the cooling system breaks and heat can’t get out of the engine compartment. This causes the temperature in the engine compartment to rise dramatically.

A quick tip: if the temperature gauge on your dashboard approaches the “H” threshold or if you see the warning lights flash, pull over as soon as you can and check what’s going on.

This article will discuss the various factors that might cause an engine to overheat, as well as how to avoid and resolve this issue.

If you find and fix the problem as soon as possible, you can save time and money on future repairs.

Top 12 Reasons Why Cars Overheat

There are various reasons why an automobile’s engine may get overheated. But if these issues are not carefully treated quickly, they might need more significant repairs in the future.

If you notice your car’s engine temperature going up, check for the following:

1. A broken thermostat

The thermostat controls the flow of the coolant. When the engine is first turned on, the thermostat valve will stay closed, stopping the coolant from moving. The valve will open as soon as the engine/coolant reaches a specific temperature. The coolant will begin to circulate and absorb the heat from the engine where it will then be passed through to the radiator to be coolant. 

Inside the thermostat are a diaphragm and a spring. Both can stick in the closed position meaning the coolant never gets to circulate inside the engine, causing it to overheat.

2. Damaged Radiator 

When the coolant passes through the radiator, it “warms up” tubes within the radiator responsible for transporting it. The fins attached to these tubes absorb the heat from coolant as it passes through. Air flows through the radiator from the cooling fan, or just passing through the radiator when driving removes the heat from these fins, thus cooling the engine as the colder coolant is pumped back around the engine. 

A radiator core is quite fragile and can get damaged by rocks/debris in the road, age-related corrosion, blocked or an impact on the front of the car. If the radiator fins or the tubes get damaged, the heat can not be fully exchanged in the car radiator, and the engine will overheat.

Additionally, the radiator cap maintains the pressure in your cooling system. If the cap is missing, broken, or leaking, the coolant can evaporate and leave the system instead of cooling it to the right level, leading to the engine’s rising temperatures.

3. Blown Head Gasket 

The head gasket in your vehicle separates the cylinder head and engine block. If the gasket fails, oil and coolant could leak into the cooling system and the combustion chamber. Head gaskets commonly fail because they are weak or the car is overheating, so there might be another problem, regardless. 

If a car overheats within a few minutes of starting the engine, you might find the issue is the head gasket. Other signs include having a low coolant level or oil found in the coolant, which is evident as a thick oil-like mixture inside the reservoir. Also, if you remove the oil cap, you may see a thick white mess attached to the oil cap, which is the coolant and oil congealed. Lastly, you may also see the exhaust pipe emits milky or a lot of white smoke.

4. A Leak in the Cooling System 

You might suspect a leak because of an empty coolant reservoir. However, this is not always the case. Leaks of coolant will often leave tell-tale puddles or spots on the ground. Depending on the coolant used, the color may be green, blue, orange, red, etc.

Leaks can appear from many places but usually from the coolant temperature sensor, the radiator, broken coolant reservoir, and pipes. Because the coolant is responsible for removing high heat from the engine bay, it is under pressure, and the boiling point of the coolant is much higher than water. If there is a leak, the coolant is no longer under pressure, and the boiling threshold will be lowered, meaning the coolant will no longer serve its purpose and cause the engine to overheat.

5. Low oil level

The engine’s oil contributes to the cooling process in several ways. The most important job of the engine oil is to prevent friction and overheating by ensuring that the different parts of the motor are lubricated. As an added aid to remove heat from the engine, the hot oil is passed through an oil cooler that works similarly to the coolant radiator.

Hence, if your car’s oil level is low or the incorrect oil viscosity is used, it poses a higher risk of overheating.

car overheating

6. Low Coolant Level 

The coolant is responsible for removing the vast majority of the heat. As soon as it absorbs heat from the engine, it goes back to the radiator for cooling. After the heat dissipates, it will circulate once again. Hence, if your cooling system does not have enough coolant, it won’t perform as effectively as it should. As a result, the engine’s temperature will go up. 

A low coolant level doesn’t always necessarily mean there is a leak, the coolant level can just be low, although its not consumed like engine oil can be so this doesn’t happen quickly. Taking a quick look at the coolant levels is as easy as checking the oil or refilling the windshield wiper fluid. Thus, we recommend doing so regularly. 

7. Water Pump 

The water pump is controlled by the cambelt/chain, so it is always in operation when the engine is running. Its job is to push the coolant in a constant cycle around the engine and then through the radiator for cooling. Even if your system has the correct amount of coolant, a faulty or leaking water pump will stop the coolant flow meaning the engine can no longer be cooled.

The most common issue with water pumps is leaking or the bearing has failed. The water should always be replaced when the cambelt is returned, then sealed inside the engine with a gasket. As with all car gaskets, they can fail, especially when left for long periods.

8. Problems with water hoses

Coolant passes to and from the engine through a series of water hoses; if the hoses become blocked or there is an airlock inside the hoses, the engine will overheat.

A blockage inside the hoses isn’t very common unless the hoses are very old and weak, in which case it’s usually the same for the radiator, so a blockage of rust and debris can occur. More commonly, an airlock in one of the water hoses can stop or reduce the coolant flow into the engine. When coolant is flushed for whatever reason, the coolant system requires bleeding to remove air.

9. Blocked heater core/ Heater matrix

The heater matrix is responsible for giving you warm air in the cabin. It works by exchanging the heat from the hot engine coolant for hot air.

A clogged heat exchanger unit can hinder the flow of coolant. Grime or dirt, and in some cars, the silica bag inside can split and cause a blockage.

Although not easy because most of the time, you will need to remove the dashboard to get to the heater core, you can sometimes remove the blockage by flushing air or water through the heater core once it’s removed from the car. 

10. Radiator fan not working

The radiator cooling fan is responsible for providing enough air to keep cool the coolant when the car is not moving or not moving fast enough for air to travel through the radiator’s core. If the fan does not operate, the engine will overheat.

The cooling fan not operating could be because of a defective temperature sensor, thermostat not opening, a problem with the serpentine belt, issue with the fan itself, or a problem with a tensioner/pulley.

Although it might not be the fan causing the overheating, it’s always best to check its operation first. Diagnosing a cooling fan problem is a bit easier than other issues. If the car doesn’t overheat when driving above 30mph, then there probably isn’t a problem with anything other than something related to the fan. You can also hear a cooling fan operating, and if it isn’t when stationary and the temperature gauge is very high, the issue may be the cooling fan.

11. Clogged catalytic converter

A clogged catalytic converter isn’t commonly associated with an engine overheating. It’s pretty rare, and usually, the driver would’ve noticed a lot of issues and chose to ignore them. When the catalytic converter gets clogged, the hot exhaust gases can no longer escape out of the muffler, forcing them back into the engine to be combusted again. This can cause the engine temperatures to rise. This and other catalytic converter problems can mean the engine will begin to misfire and stutter; this will cause the engine to struggle and overheat.

A clogged catalytic converter is usually caused by a build-up of hydrocarbons or unburnt fuel combusting inside the catalytic converter. More commonly caused by a failed O2 sensor.

12. Dirty or clogged air filter

The air filter is responsible for stopping debris from entering the engine. If the air filter becomes extremely dirty and starts to block, air can no longer pass through. This means the engine temperatures will rise because the oxygen-to-fuel ratio will be imbalanced.

Removing the filter and simply cleaning it will solve the problem; although air filters are pretty cheap, it would be better to change it.

What to Do if Your Car Overheats

If you notice steam is coming out under the hood or that the temperature gauge keeps increasing, park your car to the side and stop the engine. 

Do not immediately open the hood; you risk getting hit by hot water or steam. Wait at least 15 minutes to ensure everything has cooled down. 

Carefully open the hood and find the coolant reservoir to check the coolant level. 99% of the time, an overheating engine is caused by something related to the coolant, so if the coolant level is low or empty. Use a towel to loosen the cap and release the pressure slowly. Once opened, slowly pour the coolant to the top of the level until it reaches the maximum fill line. In an emergency, it is ok to use water but not as a long-term solution. Regardless this might only get you a mile or two before the car overheats again, and you need to pull over.

However, if the coolant level is acceptable, having your vehicle towed to a garage for repair would be better. The problem may be tricky to diagnose at the side of a road, and fixing it may be even more difficult.

Key Takeaway

An overheated engine is a clear sign that there is a significant issue. Do not attempt to continue driving the car as more damage may be done.

The most common cause for a car to overheat is a coolant-related issue or a problem with a component the coolant passes through or uses to function. Diagnosing the cause of an overheating vehicle may not be straight forward as there are so many different areas of a car that an cause the issue. This is one of the few times I strongly recommend you take the vehicle to a mechanic to get the correct diagnosis.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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