Key Takeaways

  • The first step to diagnose a cooling system leak is to visually inspect
  • The next step is to pressure test the system.
  • Repairing the leak can be easy or difficult depending on where the leak is.
  • Heater cores are a common source of coolant leaks.
  • Not all pressure testers are created equal.

This post may include affiliate links. If you make a purchase through these links, we'll receive a commission at no extra cost to you. This support helps us maintain and grow our site. Thank you for your support!

Sometimes, accidents happen. And sometimes, accidents are easily repairable, other times they are not. But what do you do when your car is leaking coolant?

The first step in diagnosing a cooling system is to pressure test the system. Pressure testing the system will show you exactly where the leak is coming from. This will allow you to see where the leak is coming from without having the car running.

I’m a mechanic with five years experience in diagnosing and repairing vehicles. I received my degree in automotive repair and I am ASE certified. I receive regular training on the most recently released automotive technology. And I repair and diagnose vehicles with the most up-to-date tools and software.

Table of Contents

Cooling System

The main job of your cooling system is to take the heat from an engine and remove it. Your engine creates a lot of heat with millions of combustions happening every so often. The heat created from combustion can be too much for the engine to handle.

Without a cooling system, the engine would overheat, and internal components would burn up. Not only would components burn up, engine oil would cook inside the engine and cause a lot of extra friction, essentially destroying the engine.

The cooling system’s job is to take the heat created by the engine and move it to the radiator. It does this through the use of a water pump. Water, or antifreeze, is pushed throughout the engine block. The heat is then transferred from the block to the water.

Then the water pump pushes the water from the engine block to the radiator. When the water is in the radiator, the water is exposed to the atmosphere air temperature which cools it down. This cooling temperature cools the engine and keeps it within normal operating temperature. This is the main job of the cooling system, to keep the engine temperature down.

How To Diagnose And Repair Car Coolant Leaking Out

A car leaking coolant is a major issue. You want to deal with the issue as soon as possible, since not dealing with it can lead to a rising temperature gauge, and in turn, a warped cylinder head. An overheated engine will give you all sorts of problems, so finding and fixing coolant leaks is a major priority.

The first thing to do in order to do a coolant leak fix is to do a visual inspection. When I do visual inspections for coolant leaks, the first places I check are the upper and lower radiator hoses. These are common places for slow leaks to happen.

The next thing I’ll do as a part of the visual inspection is to check the radiator cap. Checking the radiator cap is crucial, because if it’s not working properly, you have some issues other than a leak. The next place I’ll check is to see if there’s an issue with the radiator.

Sometimes, small holes can get into the radiator causing a coolant leak. I always check the coolant reservoir as well. Check the condition of the coolant and make sure there’s no residue or oil. If there’s oil in the reservoir, you can suspect a blown head gasket. A blown head gasket will cause a coolant leak, but it might not be so obvious as to where it’s coming from.

A blown head gasket can also cause an internal coolant leak as well, but this will cause overheating issues. After your visual inspection, if you cannot find the source of the leak, the next thing you’ll want to do is a pressure test. For pressure testing, you’re going to need some special equipment.

You’re going to need what’s called a pressure tester. You can pick up a pressure tester at an auto parts store for anywhere between $40 and $120 dollars. If you don’t plan on doing lots of pressure testing in the future, you can go with the cheaper model.

Once you have purchased the pressure tester, the next step is to hook it up to the cooling system. You usually do this by removing the radiator cap and screwing on the pressure tester adapter. The adapter is usually shaped like a radiator cap so it will spin right on.

Once the adapter is spun on, attach the pressure tester. Then start applying pressure to the cooling system. You want to bring up the coolant pressure testing system to 15psi. This will apply enough pressure to the system to mimic the car being run.

Now that enough pressure is applied to look for leaks, you’re going to want to check out the gauge on the pressure tester. If the gauge is dropping, this means the cooling system pressure is going down, and you are looking for a leak somewhere.

Take a flashlight and look high and low for a leak. Ensure that the cap to the pressure tester is on tight, since you don’t want there to be any error in your results. Somewhere there is a leak, if the gauge is dropping. Once you find the leak, start to think about how you’re going to repair the leak.

Repairing The Leak

How you fix a coolant leak is going to depend on where the coolant leak is coming from. If you’re lucky, the coolant leak will be coming from an upper radiator hose. If this is the case, all you need to do is change the radiator hose.

It’s usually a 10 minute fix that requires you to remove the hose clamps, remove the old radiator hose, install the new radiator hose, and put new hose clamps on. The same can be said for lower radiator hoses. These two hoses are common places for leaks to spring.

If the car is springing a leak from somewhere closer to the engine, you might be in for a trip to the mechanic. Since taking apart engine components at home can be difficult, you may consider taking the car to a professional, since they are equipped to deal with this.

But if you must attempt to fix it at home, start googling how to fix wherever the leak is coming from. For example, if your water pump is leaking, the instructions are going to be very different for every car in order to repair. So make sure to put your exact year, make, and model into Google when you do the search.

For example, ‘How to replace a 2012 Toyota Camry Water Pump’, and usually a YouTube video will show up with exact instructions on how to repair your vehicle. Going into instructions on how to repair a water pump is beyond the scope of this article.

If the leak is small enough, you can use stop leak, which is a chemical additive you can put in your cooling system that thickens the coolant and stops it from leaking. Though, this method is only temporary and has potential to do more damage than good. It’s best to address coolant leaks as soon as they happen, instead of waiting around to find out.

Heater Cores

Heater cores are like little radiators that live inside the cabin of your vehicle. The purpose of heater cores is to warm the inside of your cabin during cold months. Heater cores work essentially the same way radiators do. But instead of cooling down the antifreeze, the purpose of the heater core is to warm up the inside of the vehicle.

Coolant is pushed into the heater core by the water pump. Then blower motor fans live behind the heater core. Once the fans are going, they push the heat stored in the heater core into the cabin essentially warming up the vehicle. It’s quite an ingenious system since the cooling system both heats the inside of your car, and cools down the engine at the same time.

But sometimes, heater cores can spring a leak. A heater core can go bad because of a couple reasons. For example, old age can deteriorate the heater core, or the heater core can become clogged and spring a leak at one of the connections. If this is the case, you have a big job ahead of you.

To tell if you have a heater core leak, the first thing you’ll notice is a lack of heat. Without a properly functioning heater core, you’re not going to have heat. The next thing you’ll notice is a funky smell coming from your air vents. This means coolant is leaking inside your cabin somehow, and fumes are being pushed into the vents.

The last thing you’ll notice is a small leak from underneath your car or truck, but you won’t be able to pinpoint where it’s coming from. A good way to test a heater core is with a pressure tester. Hook up the pressure tester like outlined above, and then see if the gauge loses pressure.

If the gauge does lose pressure but you cannot find the leak anywhere after searching the engine compartment thoroughly, suspect the heater core. If you’ve never replaced a heater core before, and suspect it's leaking, take it to the shop.

A heater core is one of the most difficult jobs a mechanic can attempt. This is because the mechanic has to take apart the whole dashboard to get to the heater core. There are a lot of electrical components with intricate fittings on a dashboard, so it’s best to leave it to a professional.

If your car is newer, and you suspect a heater core issue, don’t even think about taking it to an independent mechanic. While independent mechanics may be more than qualified to do it, there’s no doubt they’ve never done one on your exact make and model before, if it’s a newer car.

Newer cars come with so many electronics that it’s almost impossible for an independent shop to figure out. It’s best to take an issue such as a heater core straight to the dealership. At the dealership, they have people who have documentation on how to fix it. They have mechanics who have received training directly from the manufacturer on how to fix it.

And best of all, they have insurance in case they break your car while attempting to fix it. Not to say independent shops don’t have insurance. But what I am saying is that it’s a safer option to take your car to a dealership to fix a heater core since they are properly trained, won’t need insurance, won’t break any electronics, and have thorough documentation on the issue.

Pressure Testers

Not all pressure testers are created equal, but for your use case you don’t need a top of the line pressure tester. For example, pressure testers from Snap On can range from $150-$300 dollars. Yet, if you are only purchasing a pressure tester in order to tell if you have a leak, you can purchase a cheaper pressure tester.

Tools from Harbor Freight aren’t really built for industry standards and are only meant for use a couple times. So really, you can purchase one from Harbor Freight, use it for your purposes, and return it if you want.

This would be just as effective as a mechanic who buys a pressure tester meant to be used thousands of times. When you purchase a pressure testing kit, you want to make sure that the kit has the right adapters for your car. Your car might call for the cooling system to be pressure tested on either the radiator or on the cooling reservoir. Either way, make sure the adapters will fit your car. Usually, there is a “fits your car” section underneath the tools when you research them online. This will help guide you in the right direction.

Car Coolant Leaking Out? How To Diagnose, Stop & Repair

About The Author

Charles Redding

Charles Redding

I've spent many years selling cars, working with auto detailers, mechanics, dealership service teams, quoting and researching car insurance, modding my own cars, and much more.

Read More About Charles Redding