A coolant leak can be really hard to find. In many cases, the coolant can drip off a hot part of the engine and evaporate before you can see it.

A coolant leak can cause the engine to run hot which may also damage the engine seriously, which is why it’s a good idea to fix your coolant leak as fast as possible.

How to Fix a Coolant Leak

Here is a guide of how you can find a coolant leak as fast and efficiently as possible.

1. Find the coolant leak

First, we have to find the coolant leak, of course. As I mentioned before, sometimes the coolant drips onto a hot part of the engine and evaporates, and you will have a hard time finding the leak. There could also be an internal leak that goes into the engine.

The first step in our troubleshooting is to find out whether it is an internal or external leak.

When the coolant evaporates, it often leaves a “slush” in the color of the coolant. If you have a red coolant, look for red “slush”, for example.

2. Check for external leaks

When the engine is cold, use a flashlight around and under the engine to check for signs of a new or old coolant leak. It is best to check when the engine is cold, as the coolant does not evaporate and the metal expands when the engine is hot, which could seal the leak.

If you cannot find leaks, it is best to use a tool that pressurizes the cooling system when the coolant is cold to find a leak. One tool I can recommend for this task is the Stant Cooling System And Pressure Cap Tester. I use a tool like this for all my coolant leak troubleshooting.

Normally I let the engine cool down and then pressurize the system with 2 bar. I let it rest for an hour and check the engine again. If I really don’t find any leaks and the pressure is still at 2 bar, I let it rest for 24 hours. If after this time I cannot find any external leak or pressure drop, there is probably no external leak.

Coolant Leak Tool

Before buying, make sure that the tool is suitable for your vehicle. If you cannot afford one of these tools, you can always ask a garage if you can rent one. It is a really common tool, and almost every garage has one.

You should also remember that the cooling system could get into the interior of the car and leak out of the radiator. Check the floor mat and see if it is dry or not. A leak in the radiator core can often be diagnosed from the smell inside the car. Coolant smells sweetish and makes everything sticky. If your windows mist up, this could be due to a leak in the radiator.

If you really can’t find a leak in the engine compartment or inside the car, we need to check if there is an internal leak in the engine.

RELATED: Can you use only water without any coolant in the radiator?

3. Check for internal engine leaks

Now is the time to check for internal engine leaks. Internal engine leaks often go through the engine and evaporate through the exhaust system. This creates a white, sweet smoke. If you have noticed an increased amount of white smoke from the exhaust system, that is probably where the problem lies.

If your car has an increase in white smoke and you need to top up the coolant frequently and you do not find any external leaks, you will need to do some deeper troubleshooting.

Coolant Leak

Common parts that allow coolant to leak from the exhaust system:

  1. Cracks in EGR valves
  2. Cracks in turbos
  3. Cracks in head or engine block
  4. Head gasket leaks

I will not write now about how to fix these problems in detail, because the article could be converted into an e-book! If you are interested in how to fix problems with head gaskets etc., please read our article on how to fix overheating.

A quick answer is to use the tool mentioned above. Press it to 2 bar and remove the spark plugs and remove the intake/exhaust manifold to look for leaks from cracks. Check the engine oil. If the engine oil is light brown, coolant has most likely penetrated and you should not drive the car until the problem is corrected. This can be a very time-consuming and costly troubleshooting process, and if you are unsure how to do it, get a good mechanic to help you.

4. Repair the leak

If you found your leak, now is the time to fix it. I think you need to figure out the repair procedure yourself. If not, check your service manual to see how to replace the damaged part. Always carry out your repairs as instructed and make sure that the parts you have removed are fitted with new seals.

The critical part when performing repairs on the cooling system is the subsequent bleeding. It can be really difficult to remove all air bubbles from the cooling system. If you haven’t gotten all the bubbles out and then go for a spin, your car may overheat and damage parts inside your car (not funny if you have just replaced the head gasket and then have to do it again). But don’t worry, I have written a detailed guide on how to bleed your coolant system, the easiest and fastest way.

5. Test Drive your car

Troubleshooting Car Problems

If your repair was successful, it is now time to test your repair. It is best to take the car for a long drive first. Pay attention to the coolant temperature and make sure that you do not notice any temperature peaks or overheating.

After the test drive, check all areas around the repair again for any leaks. Again, I recommend using the pressure tool and applying pressure to the cooling system for 24 hours to ensure 100% that there are no leaks.

Try the air conditioning in the car to make sure the heater is working properly.

Should I add a radiator coolant leak repair fluid into my coolant system?

I do not recommend it as it can clog the radiator core and radiator. Sure, it can repair leaks, but you need to make sure you use a really good radiator leak repair fluid. I have heard horror stories about these fluids that clogged many parts in the cooling system and caused the car to overheat. If you want to use such a fluid, I can recommend the K-Seal ST5501 Permanent Coolant Leak Repair. It is a really good fluid for these jobs, I have even used it myself in some of my own cars. But there is still the risk of clogging something if it is in bad condition from the start.

Can I add a coolant leak UV dye and check it with a UV light to find the leak?

Yes, but I can’t really see the benefits of it. Evaporated coolant often leaves traces without UV dye inside, and I don’t need UV light to detect it. You should also be careful with what you put in your coolant; you don’t want anything to crack the seals and the water pump. If you’re still interested, you can check it out on Amazon: FJC 4926 Universal Radiator Coolant Dye – 1 oz. This is not a method I can recommend, and you do it at your own risk.

My leak is from the water pump that is driven by the timing belt, do I need to change the belt as well?

You do not have to. But in most cases replacing the timing belt is a difficult and time-consuming job, so why not replace the belt at the same time? If you want to learn more about timing belts or changing timing belts, you can read our Timing Belt Guide.

I can’t find any internal or external leak, but I still have to fill coolant sometimes, what to do?

Some engines use up coolant slowly and you will not notice any white smoke. It all depends on how much you fill up. Coolant doesn’t disappear by itself; there must be an internal or external leak somewhere, even if it is only a really small leak. If it is a really small leak that you cannot find, you could try to add some of the leak stop fluid mentioned earlier.


In the meantime, I hope your troubleshooting and repair went well and you learned a lot about how to fix coolant leaks.

The pressure tool will make it much easier for you to troubleshoot coolant leaks.

Make sure that you drain all air from the system after every coolant system repair.

If you still have questions or would like to tell us how your repair went, please register and leave a comment below. I answer questions daily and you will receive a quick response.

If you liked this article and you want to know more about cars, check out our other articles about cars in our blog.