Engine Overheating – Causes, Solutions & Diagnostics

Is your engine overheated and you don’t know how to repair it, or find the problem causing the overheating?

Then this article is the right one for you. In this article we will discuss the most common causes of car engine overheating.

You will also find a guide on how to diagnose the parts that could cause your overheating. If you work through this article, I am sure you will be able to repair your car without any problems. Let’s go!

Engine Overheating Causes

The best way to make a correct diagnosis of your overheated engine is to understand how a coolant system works. First, I will discuss some parts and other possible causes that you should investigate if overheating occurs. Later in the article you will find out how to diagnose the parts to replace the correct one.

The most common causes of engine overheating are a faulty thermostat, a faulty CTS sensor, air locks in the cooling system, a faulty radiator fan, a faulty water pump, or a defective head gasket. You should always diagnose your car correctly and not just replace parts. Get the help of a professional mechanic if you are not sure how to diagnose your car correctly.

Low Coolant level

In most cases, a low coolant level leads to overheating of the engine. However, if the coolant level has become too low, you most likely have an internal or external coolant leakage from your engine. If the cooling system is working properly, you should never need to top up coolant. If the coolant level has become too low, you may need to drain the air from the cooling system after topping up.

Clogged Radiator

A clogged radiator causes the coolant to not flow properly through the engine and can cause the engine to overheat. This is not very common and I recommend checking the other parts first. The radiator is most likely mounted in front of the engine.

Faulty Thermostat

The thermostat is often located in the rear coolant hose on the engine, but may also be located on the upper coolant hose depending on the vehicle model. The thermostat should open at 90°C to maintain the same temperature when the engine is hot. It happens very often that the thermostat fails, so I can recommend to check it first.

Failed Water Pump

The water pump pumps around the coolant to ensure a constant flow to cool all parts and prevent the engine from overheating. Sometimes the water pump is driven by the timing belt and sometimes by the serpentine belt. In some situations you also have electric water pumps. A failed water pump will cause the engine to overheat and it is not always easy to diagnose a failed water pump correctly.

Expansion tank/lid

In addition, a defective expansion tank or the lid can also lead to overheating if it leaks or is blocked somewhere. If the expansion tank leaks, it can force coolant out and create air in the cooling system, which will cause the engine to overheat. The expansion cap often also has a pressure valve that ensures that the cooling system maintains a high pressure in the cooling system, about 1-1.5 bar. Higher pressure increases the boiling temperature and helps to prevent the engine from overheating. A faulty cap can lead to too low pressure and cause overheating.

Coolant/Radiator Fan

The coolant radiator fan has the task of cooling the coolant flowing through the radiator. However, when driving at higher speeds, the slipstream helps to cool the coolant and the fan is not needed. However, if you notice that your coolant temperature increases at idle and lower speeds, there could be a problem with your coolant fan.

Coolant Temperature Sensor

A coolant temperature sensor may not cause the engine to overheat on its own, but if your vehicle has an electric thermostat or water pump, it may cause it to malfunction. A faulty coolant temperature sensor can also trick you into believing that your car is overheating, which is not the case.

Blown Head Gasket

A blown head gasket can force air or compression into the cooling system and cause airlocks, causing your coolant to overheat. It can also cause engine oil to mix with the coolant, causing restricted flow. A blown cylinder head gasket is often an expensive repair because you have to remove the engine cylinder head.

Air in the coolant system

Airlocks in the cooling system often lead to overheating of the engine. However, the air will not enter the cooling system if you have not replaced any parts or have other defective parts on the engine. Either way, it is a simple step to bleed the air from the engine to see if it works. If you want to learn more about bleeding your cooling system. Take a look at this article: How to bleed air from a car engine.

These are the most commonly mentioned parts in a cooling system. Of course, depending on the car model, there may be other parts, but these are the most common causes of engine overheating.

Overheating Repair/Diagnose/Fix.

Now that we know a little more about how the coolant system works, it is time to learn how to correctly diagnose these possible causes. Here are the things to look for when trying to repair your overheated engine.

1. Check your thermostat

If the car is overheated, you can check whether the thermostat has been opened or not. To check whether it opens, you can feel the large lower or upper coolant hose, depending on where the thermostat is located. If the hose is cold when the engine is overheated and you have already tried to bleed the cooling system, you probably have a problem with your thermostat. In this case, replace the thermostat. You can also remove your thermostat and place it in a pot of hot water to see if it opens and closes properly. But a new thermostat is not that expensive and it is often a lot of work to remove it.

NOTE: Some vehicles use an electric thermostat and you should always check the DTC fault code memory first with an OBD2 scanner for related fault codes.

There is another, much safer way to check this. It consists of using a laser temperature tool. Simply shoot the laser at the hose from a safe distance and it will show you the temperature. If you want to buy this tool, I could recommend one of these: Etekcity Lasergrip Digital Laser Infrared Thermometer

2. Check your Radiator fan:

If your thermostat is working properly and you haven’t noticed any strange drops in temperature on the hoses, it’s time to check the radiator fan and make sure it’s working properly. The most common symptom of a failed radiator fan is that the vehicle will overheat when idling or at lower speeds because the vehicle does not rely on the radiator fan at higher speeds.

You should check that the radiator fan is rotating at full power with the thermostat fully open and the engine hot or overheating. If the radiator fan does not work, you must ensure that the temperature sensor for the fan is working and you can also ensure that no fuses have blown. If you have tested or replaced your thermostat and the car is still overheating, it is time to move on to the next step.

3. Check your water pump

Now it is time to check whether the water pump is delivering water. It can be very difficult to diagnose a water pump without removing it. Some quick checks can be felt on the water pump axis to see if it can be reached. Any strange noises? Is the backslash okay? An example: In some Volkswagen engines, the water pump uses a plastic pump unit and the water pump works normally as usual. But when the engine gets warm, the plastic expands and stops pumping water. It can be really difficult to diagnose because you don’t feel anything strange with the water pump.

As I said, it can be really hard to diagnose. The only way is to try to see if there is any flow in the tubes or to remove the pump to check. You can also try using a small camera to see if you can use it to reach the inside of the pump. If you have checked the water pump and the car is still overheated, continue with the next step.

4. Check the head gasket

If your car only worked well for a short period of time after the air was released from the cooling system, but after a while the same problem occurred again, it could be your blown cylinder head gasket that is forcing compression or air into the cooling system.

The easiest way to test this is to see if there is very high pressure in the cooling system and if this pressure is constantly bubbling out of the cap. You could also go to a garage and see if you can borrow their exhaust tester.

Place the exhaust tester near the coolant cap to see if you get high CO values there. You can also use your sense of smell to see if you can smell exhaust fumes in the expansion tank or radiator. Another way to see this is to check if oil and coolant have mixed, this is a good sign for a blown cylinder head gasket.

To learn more about blown head gaskets, read our other article, blown head gasket symptoms. If you have high values and high pressure within the coolant system very quickly: Replace the head gasket and check the head for cracks.

5. Check if the engine is really overheating at all

Now you have checked almost every part of a coolant system. But the question is, is the car really overheating or is the dashboard showing the wrong information?

If the water gets too hot and explodes through your cooling system to the outside, that’s obviously not your case. But if the only thing you’ve noticed is that the temperature on your dashboard is rising, then there could be a problem with the dashboard coolant sensor. Some cars use a temperature sensor for the engine control unit and another sensor for the dashboard. You need to find out where your dashboard gets its information from.

If your car uses one sensor for both the dashboard and the engine ECU, you can check the live data from the sensor using a diagnostic tool.

6. Flush your coolant system

If you have checked the water pump and thermostat and have tried several times to remove the air from the pump, there may only be one thing left if you have checked all parts correctly. The flow in the cooling water system must be throttled or blocked. The problem will most likely be in your radiator or in another coolant hose on the cooling system, so I recommend that you start checking.

If you wish, you can try flushing your entire coolant system. There could be a lot of dirt in it and trying to flush it might be a good idea because it’s really cheap to try, and even if it’s not the problem, it won’t hurt to do it anyway.

If you don’t know how to flush your cooling system you should watch this video. I can really recommend it:

Now I really hope that your overheating problem is solved. If not, I recommend you to start over with the guide and double check everything. If you have tried everything as I mentioned above and the car is still overheating, you can contact us and write a comment below and we will try to help you. Explain your problem and tell us what solutions you have already tried to give you a reliable and quick answer!

Conclusion

It is not always easy to diagnose parts within a cooling system without removing them and inspecting them with your own eyes. The good thing about it is that you have learned how a coolant system works and which parts are contained in it. You will find that there are not many different parts that could fail. The coolant system is a really simple design that works very well. If your car uses electrical parts in the cooling system, such as an electric thermostat or electric water pump, you should always read your DTC fault code memory first to see if the engine control unit registers problems.

I hope you have read and enjoyed this article about overheating problems. If you like this article or if you want us to change or add something, you can comment below. If this article did not really help you with your error, you can ask us a question on our homepage. Write your question as detailed as possible and we will certainly give you a detailed answer.

See you in the next article!

Magnus Sellén

Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.