coolant overheating

How to fix an Overheating car engine

In Coolant by Josh SLeave a Comment

overheating water pumpDo you have an overheating issue with your car engine? Are you afraid that the head gasket is blown? It’s a very common thing that people thinks their head gasket is gone and they are selling the car for really cheap.

To be honest I have bought some of these cars myself. A blown head gasket is not a very common problem at all. Read through this article and I’ll explain to you step by step of how you can diagnose your coolant overheating problems. Let’s start!



I work as a diagnostic technician and my job is to solve the most advanced problems in vehicles. In this article, I will share my knowledge for free with you. I will learn you how a coolant system is working and how you can diagnose and fix it in the easiest way. There are a lot of tips that I will share with you in this guide about how to fix an overheating engine. If you are going to fix your overheating engine, you have to know how the system works before you start your diagnosing. I will explain how you should do that like a professional in the next caption.


How an engine coolant system works

The best way to make a proper diagnose for your overheating engine is to learn how a coolant system works. You will probably hear from your friends or mechanic of some tips they had, maybe they are telling you about what part they were replacing 10 years ago when they had almost the same problem. Sure, they can guess right. But in engine coolant systems, there are so many parts that could make an overheating problem so you can’t just guess what you think is the problem. It could be very time-consuming and expensive if you are unlucky.

So, to do a good diagnose we have to learn how a coolant system works.

coolant system

This is how a typical coolant system is working. The coolant systems could look a bit different depending on what car engine you have, but the principle is the same in almost all engines. So don’t worry if you have an older car without the expansion tank, that is showed in this picture. You will understand what we are talking about anyways.

The water pump is pumping coolant around in the engine and to the heater core when the thermostat is closed. When the thermostat is open, the water pump is pumping around water through the radiator also.

When the engine is cold, the thermostat is closed so that the water would not flow through the radiator. The water pump is just pumping around the coolant inside the engine around and around till the coolant is getting warm.
The thermostat is mounted on the engine and it starts to open at around 90 Degrees Celsius. 200 ~ Fahrenheit.
The fan on the radiator is cooling the coolant down before it is going back into the engine again.

When the coolant started to get warm the thermostat will open up a little bit and in the beginning, just let a little water flow through the radiator. When the coolant system starts to get warm it will flow more and more.


coolant system

This is how a common thermostat is looking. It uses some springs and wax, yes wax. The small part in the middle of the thermostat is containing wax. When the wax is starting to melt at the given temperature (90c in this case),  the part is expanding and opening the thermostat up and the coolant can then flow through the radiator.

At this picture, you see a common water pump. The water pump is usually driven by the serpentine belt, but in many new cars, it is driven by the timing belt. Now when you got some more knowledge about how the coolant system is working, we can go to the next step of how to diagnose a coolant overheating.


How to fix an Overheating car engine.

Now when we know a bit more about how a coolant system works, it is time to think out of what parts could causing the overheating. We can start to look at the same picture as before again:

coolant system

What could cause an overheating? The most likely thing to cause an overheating is that we do not get flow through the radiator and that the coolant is stuck inside the engine. So if we are looking at the picture we could say that the possible causes for an overheating are:

  • Water pump is not pumping around the coolant
  • Thermostat is closed all the time
  • Radiator or hoses is not letting coolant through, a lot of dirt
  • Air in the system, making the coolant to get stuck.
  • The radiator fan is not working

Now we know the possible causes, but where do we start? A very common problem is air in the system if the coolant level has been low at any time or you replaced any parts. If you know that the level was low or the overheating was happening after any repair, I recommend starting to get all the air out. It could also be a blown head gasket that is pushing air into the system. I will explain how to check it the best way now.

How to get air out from the coolant system.

To get all the air out of a coolant system can often be really, but there are some tricks that will make it much easier.

  1. First, you have to find any air screws on your coolant system
  2. Raise the front up as high as possible with a screw jack or go to a steep hill.
  3. Make sure the coolant is cold before you are doing this
  4. Remove the coolant cap or any airscrews
  5. Start the engine and rev it up to around 2000 RPM.
  6. Be prepared to fill more coolant at all the time if the level gets low, can be good to bring a friend that is doing that while you rev it up.
  7. Do it until you see that the coolant level is not going down anymore.
  8. Put the coolant cap and the airscrews back and take the car for a test drive.

Possible head gasket problems:

If your car was just working good after this process, but after a while, the same problem came back, there could be your head gasket that is pushing compression/air into the coolant system. The easiest way to test this is to see if you get very high pressure inside the coolant system and it’s bubbling out from the cap all the time. You could also go to a car workshop and check if you can lend their exhaust tester. Put the exhaust tester near the coolant cap to see if you get high values of CO there. If you got high values and high pressure inside coolant system very fast: Replace head gasket and check for cracks in the head. Now if your car is still overheating directly after you tried these things, it’s time to go to the next step.


Check your thermostat

When the car is overheating, you can check if the thermostat has opened or not. To check if it is opening you can feel on the big lower or upper coolant hose, depending on where the thermostat is fitted. If the hose is cold and you have tried to get the air out, you probably have an issue with your thermostat. In this case, Replace the thermostat. You can also take out your thermostat and put it in a saucepan with hot water to see if it is opening. But a new thermostat is not that expensive and it is often much work to remove it. If you are going to do this, you have to be really careful so not any hose will explode and you get over 100 c coolant water on you.

NOTE: Some vehicles are using an electric thermostat, and you should always check the DTC trouble code memory.

There is another, much safer way to check this. It is to use a laser temperature tool. Just shoot with the laser on the hose from a safe distance, and it will display the temperature for you. If you want to buy this tool, I could recommend one of those:

There could always be another problem like stop or air in the system or no flow from the water pump. But a faulty thermostat is a really common problem and to test it out to be 100% sure, you either have to remove your thermostat and try it out or replace it.

Check your Radiator fan:

Next step, if your thermostat is working and the hose gets warm is to check if your radiator fan is working properly. You should check if the radiator fan is spinning when the thermostat is fully opened and the car is warm. If the radiator fan is not working, I recommend you to check out this video.


When you have tested or replaced your thermostat and the car is still overheating, it’s time to go to the next step.

Check your water pump

Now it is time to check if the water pump is flowing any water. It can be really hard to diagnose a water pump without removing it. Some fast checks are to feel on the water pump-axle if you can reach it. Any strange noises? Is the backslash okay? For an example: some Volkswagen engines, the water pump is using a plastic pumping unit, and the water pump is working as usual normally. But when the engine is getting warm the plastic expands and it stops to pump water. It can be really hard to diagnose because you can’t feel anything strange with the water pump.

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


Check if the engine is really overheating at all

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

Of course, if the water gets too hot and exploding out through your coolant system, this will not be your case. But if the only thing you have noticed is that the temperature on your dashboard is raising then there could be a problem with the coolant sensor to the dashboard. Some cars are using one temperature sensor for the engine control unit, and another sensor for the dashboard. You have to find out where your dashboard gets your information from.

If your car is using one sensor for both the dashboard and the engine control unit, you can check the live data of the sensor with a diagnostic tool.


coolant overheating

Flush your coolant system

If you have checked the water pump, the thermostat and tried to get the air out from the several times. There could only be one thing left if you have been tested all parts correctly, and that is: The flow in the coolant system has to be restricted or blocked. The problem will most likely be in your radiator, So I recommend you to begin to check it.

If you want you could try to flush your whole coolant system. There could be a lot of stuck dirt inside it, and to try to flush it out could be a good idea because it is really cheap to try and even if it’s not the problem it wont hurt to do it anyways.

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

Now I really hope that your overheating problem is fixed. If it is not then I recommend you to start over with the guide and double check everything. If you tried everything as I mentioned above and the car is still overheating. You can contact us and Ask us a question. Explain your problem and explain what solutions you have tried already to make it much easier to give you a reliable and fast answer!



To diagnose parts inside a coolant system is not always that simple without removing them and inspect them with your eyes. The good thing about this, when you have learned how a coolant system works and what parts are included. You will realise that there is not that many parts that could fail. The coolant system is a really simple construction that is working very well. If your car is using any electrical parts in the coolant system, like an electric thermostat or electric water pump. Then you should always read your DTC trouble code memory first to see if the engine control unit is registering any problems.

I hope that you learned and enjoyed this article about overheating problems. If you like this article or want us to change or add something, you could comment down below. If this article did not really help you with your fault, then you can post a question to us at our homepage. Write your question as detailed as possible and we will for sure give you a detailed answer.

See you in the next article!

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