Coolant Leaks: What Causes Them, and What to Do

Stephen Fogel
February 2, 2018

coolant leak

Your car’s cooling system is vital to the engine’s health. It keeps your engine at an optimal temperature, which helps it run smoothly, improves fuel economy and ensures proper functioning of the emissions system. It also helps keep you cool when you turn on the air conditioner, or warm when you hit the heat. 

But your car can develop a coolant leak. These leaks can be minor or major. If your car is losing coolant, your engine can overheat, resulting in serious engine damage, followed by very expensive repairs. If the damage is bad enough, your engine could be completely destroyed.

Let’s look at how the cooling system works, reasons your car might have a coolant leak, and what to do about it.

What your coolant does

The cooling system keeps your engine from overheating by circulating coolant through it. As the coolant moves, it absorbs the intense heat generated by the combustion process. The coolant — a mixture of antifreeze and water — then travels through your radiator, where it releases that heat.

Your coolant also performs other protective duties for your engine as it circulates. Its alcohol content protects against freezing at low temperatures. Special additives in your coolant prevent corrosion, rust and mineral deposits from forming along the pathways of your cooling system.

Coolant vs. antifreeze: What’s the difference?

When reading about your vehicle’s cooling system, you come across these two terms, usually used interchangeably. But do they mean the exact same thing? Not necessarily. Strictly speaking, antifreeze is the substance that is added to your cooling system to keep it flowing at below-freezing temperatures, hence the name. But it also raises the boiling point so that it can continue to operate in your engine and absorb heat at above-boiling temperatures. This gives your car year-round protection. Antifreeze also has additives that prevent corrosion and scale buildup within your cooling system.

When the antifreeze in the plastic jug is mixed in the recommended 50-50 proportions with distilled water, you now have coolant. This is the mixture that circulates through your engine and radiator, absorbing the heat of combustion and releasing it to the atmosphere. There you have it — antifreeze is half of your coolant, but not all of it. The rest is water.

If you notice a leak, it’s important to get it repaired. An engine failure while you’re driving will usually result in a breakdown by the side of the road, which is both a headache and a hazard. 

IMPORTANT: If your temperature gauge goes near or into the red zone, or your temperature warning light comes on, this could be a sign that your engine is about to overheat. Overheating carries very serious mechanical and financial consequences. Turn your A/C off and your heater on full blast, as this will help to lower the engine temperature. Then, as soon as you safely can, get off the road and shut off your car. Call your mechanic for an opinion on what to do next. Do not open the hood to check anything until the vehicle cools down — you could sustain serious injuries, including burns.

Verifying you have a coolant leak

If your in-dash temperature gauge is running hot, this can be an indicator of a coolant leak. The temperature gauge monitors your coolant temperature. If there’s a leak, that means that there’s less coolant in your system to get the job done. The remaining amount of coolant now has to absorb more heat per unit of volume, resulting in a higher coolant temperature.

Your first step is to verify that there is a coolant leak, and then to determine how bad the leak is. Some leaks come from the various parts of the cooling system, and allow coolant to drip onto the engine, or the ground under your engine. Other coolant leaks can happen internally and require other means of detection.

Cooling systems are set up with an expansion tank that connects to the radiator. The tank holds extra coolant that can expand or contract as the engine heats and cools. This ensures the proper amount of coolant is available under all conditions. The expansion tank is also where you can check and top up the coolant level. It will have marks on it showing you the minimum and maximum levels of coolant that should be in it.

To check your vehicle’s coolant level, park it on a level surface and check it when the engine is cold. The level in the tank should be between the minimum and the maximum. If it’s below the minimum, top it up with a mixture of 50% antifreeze and 50% distilled water. You can mix it yourself or purchase premixed coolant.

Now that you’ve established a baseline coolant level, check it every day to see whether there are changes. If the level drops significantly, top it up and make an appointment with your mechanic immediately. You have a major leak.

Get it diagnosed by a professional

Identifying an external coolant leak

Check under the vehicle: If you see a green or orange liquid under your engine on your garage floor, driveway or parking spot, this is likely a sign of a coolant leak. You may also smell the sweet-ish odor of coolant when you get out of your car after driving it — this could indicate coolant leaking onto hot engine parts.

Another good way to determine the amount and location of the external coolant leakage is to place a large piece of cardboard under your engine when you park it overnight. Check it the next morning to see how much coolant has leaked out, and where it’s coming from. This is very useful information to have for tracking down the source of the leak.

Check under the hood: Do this first when the engine is cold and the car is off. Open the hood and look around for signs of a coolant leak. Use a flashlight and check for leaks and drips coming from the radiator, cracks or wetness on the hoses attached to the top and bottom of the radiator, the smaller heater hoses, and the hose connections. You may also see rust spots where the radiator is leaking. Check the end of the engine where the drive belts are attached; this is where the water pump resides, and it can leak coolant. Inspect and squeeze the hoses you can reach — they should feel firm and not spongy, and be free of cracks. Check around the sides of the engine to see if there is any coolant leaking from a head gasket.

Now start the engine, let it warm up, turn it off and check again, being careful to stay away from the hot engine parts. Some coolant leaks will only occur when the cooling system is warmed up and pressurized. Check the radiator (particularly along the seams), hoses, connections and other areas listed above. Wear safety glasses or goggles to protect your eyes.

Identifying an internal coolant leak

While most external leaks can be fixed pretty painlessly, internal coolant leaks are a different story. They’re often caused by a failure of a head gasket, which is supposed to allow the coolant and the oil to circulate through the engine on completely separate paths. Head gasket failure can let these fluids go where the other normally travels, or into the combustion chamber itself. And that’s a recipe for engine disaster.

If you’re losing coolant, but see no evidence of a leak: You may have coolant leaking into the engine cylinders, where it’s being burned along with the fuel. A sign this is happening is a burst of white smoke from the tailpipe when you start the engine in the morning. This is bad for both your engine and your catalytic converter.  

If your oil looks very strange: If your oil is a frothy, white or milk chocolate-colored substance on the dipstick, it’s likely that a faulty head gasket has allowed coolant to be mixed with the oil. On some vehicles, a faulty oil cooler can also be the culprit. Coolant is not a good lubricant, so this is also a very bad thing. Call your mechanic immediately. 

Common sources and causes of coolant leaks

There are many places where a coolant leak can occur. These are the most likely culprits.


Your radiator is a pressurized device made up of many different parts. It’s exposed to the airflow that enters the front of your vehicle. It’s also exposed to stones and debris that are thrown up by other vehicles, vibration and shocks from rough roads, and the constant internal flow of hot, high-pressure coolant. Radiators are metal with plastic tanks attached on the sides, and it's this attachment area where leaks can occur. They can also rust, and be subject to metal fatigue and wear on high-mileage cars.  

Radiator cap

Your radiator cap sits on top of your radiator or its expansion tank and can be removed to check the coolant level. But don’t touch it when the engine is hot! When working properly, the radiator cap provides a tight seal that helps keep the cooling system pressurized. Over time, the cap, its spring or its seal can deteriorate. It will then be unable to hold the correct pressure, and can be the source of a coolant leak.

Radiator and heater hoses

These hoses carry coolant from the engine and the heater core to the radiator and back again. They can age, become brittle, crack, split, kink, bulge or develop holes, particularly at places where the hoses bend. They can also loosen where they attach (with clamps or other fittings) to the radiator, engine or heater core. These can all be sources of coolant leaks. If you replace a hose due to old age, it's a good practice to replace them all to prevent future leaks and possible overheating. 

Expansion tank

This plastic container holds extra coolant, which goes in and out of the radiator as the engine gets hotter or cooler. It’s usually connected to the top of the radiator by a piece of rubber hose. The container can crack, the cap can leak, the hose can deteriorate, and the connections between them can get loose. All these situations can result in a leak.

Water pump

Your water pump’s job is to circulate the coolant throughout the cooling system. Without a functioning water pump, your engine will quickly overheat. The water pump is normally driven by a belt, and can be found low on the part of the engine where the drive belts are located. It usually connects to the radiator through its lower hose. Failure of the water pump bearing seal or its gasket, or external damage, can cause a coolant leak.


Your thermostat is the component that regulates the temperature of your coolant, keeping it at a point where the engine runs most efficiently. It opens when the coolant is warm, allowing the coolant to flow through the radiator. It closes when your coolant is cold, letting the coolant be warmed quickly by the engine’s heat. The thermostat housing can crack, or its gasket may fail. Either scenario can end up giving you a coolant leak.

Heater core

Your heater core is a small, radiator-like component that is located behind the dashboard. Its function is to provide heat to your vehicle’s interior. The heater core receives warm coolant through its own set of hoses. If there is a defect or damage in the core, its hoses or its connections, this can be a source of a coolant leak. You may see coolant in the footwell, leaking from under the dash. You may also notice a sweet smell from inside the vehicle, and the windshield can fog up with moisture. 

Intake manifold gasket

The intake manifold is located on the top or on the side of your engine. It provides a mixture of air and fuel to the cylinders, where it’s burned to provide power. The intake manifold can have engine coolant flowing through passages in it and the intake manifold gasket can fail and leak coolant.

Head gasket

Your head gasket is a crucial part of your engine. It provides a seal between the engine block and the cylinder heads. The head gasket helps to retain the pressure of the combustion process in each cylinder. It also allows the flow of both oil and coolant through their own individual passageways to lubricate and cool the engine. 

If the head gasket fails, it can cause several different problems. You may experience poor engine performance, rough running, stalling or stuttering. You may notice white exhaust smoke and the engine may run hot or overheat.

Head gasket failure can also lead to a breach of the separate oil and coolant passages, causing a situation where the oil and coolant mix with each other. This can give you oil in your coolant, coolant in your oil, or coolant in your combustion chambers. All of these are bad.

If the head gasket fails where it’s exposed to the exterior of the engine, you may be able to find a leak between the block and the heads, possibly under the exhaust manifold. But because the effects of a bad head gasket often occur internally, you may not see any leaking coolant. Everything can be happening inside the engine, which requires a different strategy. Finding a mixture of oil and coolant is a sign that your head gasket has failed. 

What you can do about coolant leaks

Coolant leaks should be dealt with promptly — the continued good health of your engine depends on it. 

Some of these fixes are straightforward, some require a little mechanical knowledge or experience, and some are very complicated. For anything you’re not sure you can tackle, take your car to a certified mechanic for a thorough diagnosis.

Fixing a leaking radiator

Replacement radiators are readily available for most newer cars. The replacement process involves removing the bad radiator and replacing it with a new one. The cost can be reasonable, compared to replacing other major mechanical components.

Older cars may need to have their radiators rebuilt, if a replacement radiator is unavailable or very expensive. This process takes some time, so your vehicle will be out of commission for a while.

Fixing a bad radiator cap

Replacements are inexpensive. Be sure that your replacement cap will fit your vehicle, and that it has the proper pressure rating.

Fixing bad radiator and heater hoses

If your hoses are leaking at their ends where they are clamped on, first try tightening the clamps and see if that solves the problem. If the hose is clamped to a plastic coupling or radiator neck, be very careful not to overtighten and crack the plastic connection. If this doesn’t work, it’s time to replace the bad hoses.

Old, leaky hoses can be removed and replaced with fresh new ones pretty easily, but be aware that you may have to drain the coolant from your engine first, depending on which hoses you’re replacing. This might be a great opportunity to flush your cooling system and replace your coolant, if it’s been in use for a long time. Your mechanic can handle this easily. 

Fixing a leaking expansion tank

If this plastic container cracks and develops a leak, the only solution is to replace it. The part is easy to find, and it’s a fairly straightforward process, but be sure you have the right tools to do it. Otherwise, leave it to your mechanic.

Fixing a bad water pump

Water pumps that go bad are usually worn out to some extent, so the best long-term solution is a replacement pump. Leave this to the professionals.

Fixing a bad thermostat

You need a replacement thermostat. These are widely available and inexpensive. If you have some mechanical experience, you might be able to handle this repair; otherwise, let your mechanic do it.

Replacing a bad heater core

This is a major project, as your heater core is buried under the dashboard. There can be lots of labor involved here, and it’s not a job for beginners. 

Fixing the intake manifold gasket

Replacing the intake manifold gasket is more straightforward, but requires some engine disassembly. It’s best left to a mechanic.

Fixing a bad head gasket

This is the big enchilada. Your engine will need to be partially disassembled, have the gasket replaced, and then be reassembled. Other processes may be necessary if the engine has overheated.

It’s an expensive repair, and you’ll have to decide whether it’s worth it or if you should get another car instead. Get an estimate from your mechanic for the head gasket replacement. Then figure out what your vehicle will be worth, both with and without a functioning engine. If you have an older vehicle with lots of miles on it, it may not be worth fixing.

One bit of good news: If you have a newer vehicle, the repair may be covered under your powertrain warranty. Check your warranty booklet to verify your coverage. If it’s not, run the numbers. It may be worth fixing, based on the current value of your vehicle. Your mechanic can help you work through this, so have the conversation.

Stephen Fogel

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.