Coolant leak on a car

Car Coolant Leaks: How To Find And Fix Them

Coolant leaks are one of the most stressful problems you can have with your car. A coolant leak can happen at any time, and you simply can’t drive a car having such a problem. If you do so, sooner or later, the engine will overheat. Unfortunately, overheating can quickly cause your engine to seize and, if that happens, it will need to be replaced. The good news is that finding and fixing car coolant leaks isn’t hard at all. In this article, we’ll explain how the cooling system works, where it’s most susceptible to leak, how to find and locate a leak and what you can do to fix it.

Car cooling system

How Your Cooling System Works

Your car’s cooling system includes the radiator, the water pump, the heater core, the thermostat, and the radiator hoses. The water pump circulates the coolant through the engine. Within the engine, numerous passages allow coolant to run around the hottest part. The temperature will then be transferred to the coolant which will return to the radiator to be cooled down with some help from the radiator fan.

While the cooling system seems quite simple, it’s proper operation relies on many important factors. Positive pressure needs to build up in the system so the coolant doesn’t reach the boiling point at 100 degrees Celsius. That is taken care of by the radiator cap. However, if there’s a leak somewhere, the system won’t be able to build up and might not work properly.

How to Identify Coolant Fluid

Before trying to identify a leaking component in your coolant system, it’s important to make sure that the fluid under your car is actually coolant fluid. It’s not uncommon to notice liquid on the ground under the engine bay after driving for a while and leaving the car parked somewhere. And it’s not always wrong.

If your car has air conditioning, you may be seeing condensation water pooling. This is absolutely normal.

Other automotive fluids coming from a totally different system might also be leaking. If the puddle is bright green, bright orange, pink, yellow or blue, it may very well be coolant. However, transmission fluid is also red and can sometimes be confused with coolant fluid. So how are you supposed to know you if the fluid under your car is truly coolant?

The first thing to do is to verify the coolant overflow tank under your hood. If the tank is empty, it’s very likely coolant. You can also touch it with your fingers. Coolant will feel like water. Transmission fluid would feel greasy to the touch.

Smelling it might also help. Transmission fluid often smells horrible while coolant smells sweet.

All that to say that you should always confirm what fluid is under your car before starting to troubleshoot the problem. Trust me, performing this step might save you a lot of time in the future.

Used and new radiator hoses

Finding and Fixing Coolant Leaks

Radiator hoses

One of the most common places to find coolant leaks are from the hoses. Radiator hoses come in pair. Every car has an upper and a lower radiator hose. These are the large, molded rubber hoses that go from the radiator to the water pump and from the engine to the radiator.

Car coolant leaks commonly happen when the fasteners or hose clips become loose or wear out. Rubber also dries out and cracks over time and may start to leak.

You should inspect your hoses at least several times each year. It is always a good idea to make sure that your cooling system is in good condition before the higher temperatures of summer begin. However, we would recommend you to put hoses inspection on your regular maintenance checkup list. If you bring your car to a mechanic for oil changes and maintenance, then no need to worry. Inspecting cooling system components and looking out for leaks is always part of inspection included with your oil changes.

When you look at your radiator hoses, you should look for obvious signs of damage, like cracks and leaks. You can identify worn radiator hoses because they will be swollen and feel spongy.

Car heater core

Heater core leaks

The same thing may also happen with the heater hoses. Heater hoses frequently fail during the winter when they are being used more often. When you inspect your heater hoses, look for cracks and any other signs of trouble. Heater hoses can get too close to hot parts of the engine and start leaking. If you find coolant leaking inside your car, often on the passenger footwell, you can be certain that you have a leak at the heater core. Inspect the heater core hoses and the heater fins for crack and leaks.

Radiator leaks

Radiators will most often leak at the top, bottom or on the sides. Everywhere where there’s a weld, in fact. You may also see radiator leaks where the hoses attach. When coolant leaks through the radiator fins, the coolant will dry out and stain the fins. In that case, you’ll probably have to replace your radiator.

Radiator car coolant leak

Plastic radiators, unlike metal ones, are molded into shape instead of welded. In that case, radiator casing leaks are a lot less frequent.

Furthermore, most radiators have a small drain valve on the bottom. Sometimes, this valve can become unintentionally opened or the rubber seal may start leaking. If the valve is leaking, replacing the seal will usually do the trick.

Water pump leaks

Water pumps often have a weep hole that allows coolant to leak out when the seals fail. The leak could be just a little dribble that you hardly notice, or it could be a substantial leak.

Coolant leaks also happen around the sides of the water pump where it mounts to the engine when the gasket fails.

The pipes that connect the water pump to the heater hoses can also become corroded and allow leaks. In this situation, replacing the water pump is often the only thing you can do.

Engine block leaks

Your engine block has several plugs on the sides commonly called freeze or frost plugs. Freeze plugs are designed to pop out of the engine block in the case where the coolant was to freeze. Coolant doesn’t typically freeze but it could happen if the wrong coolant-water mix was used. Without the freeze plugs, your engine block would crack if the coolant was to freeze.

Finding car coolant leaks from freeze plugs is quite difficult to fix. Often, you will have to remove the engine from your car to replace the freeze plugs and fix the leak. The plugs must be driven in to make sure they seat right and it’s usually not possible while the engine is sitting in the car. It’s important to mention that I’ve never encountered a freeze plug problem before in my life but it can happen.

Most often, when there’s coolant leaking from a freeze plug, it’s, in fact, leaking from an aftermarket block heater. Block heaters are frequently installed in the place of one of the freeze plugs and it’s quite common to see them leak after a couple of years. When it happens, the easiest thing to do is to simply replace the block heater. That’s usually a lot simpler than trying to remove it and reinstall a new freeze plug.

Mechanic holding a leaking head gasket

Internal leaks

The passages inside your engine that allow coolant to circulate must pass through several different parts that are sealed with gaskets. A faulty cylinder head gasket is the most common cause of internal coolant leaks. When the gasket fails, the coolant can pass into the cylinders, causing serious engine damage. The intake manifold and the cylinder heads also have coolant passages for coolant that run to the engine block.

Coolant may also contaminate the engine oil, significantly reducing its lubricating properties. In most internal leak cases, you won’t notice any signs of a leak. If only that the coolant level will decrease in the overflow tank. If the level goes down and you can’t find coolant on the floor, you can begin to suspect an internal coolant leak.

How to Fix Car Coolant Leaks

You should always carry a small tool kit in your car that will allow you to make quick roadside repairs. Being able to tighten up loose hose connections on the road may someday save you from being left stranded somewhere.

Unfortunately, you can’t drive a car with leaks or holes in the radiator hoses. The pressure inside the system will simply spray the coolant out and will drain the system in a short lapse of time. Even if it’s only a small hole or crack, you never know when the pressure will cause the hose to rupture completely.

If it happens while you’re driving, using waterproof tape or instant sealants sold at your local auto parts stores might temporarily fix a radiator hose. However, you’d still need to head to the closes repair shop right away. This type of repair usually doesn’t last long.

Another option is to use flex-style universal radiator hoses sold at local auto parts stores. These can be particularly handy when trying to fit a radiator hose to a vehicle that doesn’t have the stock engine or radiator.

If your car has a heater core leak, you can temporarily bypass the heater hoses by connecting the inlet to the outlet. Just remember that while this trick may get you out of trouble, the heating system will not work anymore.

You can’t ignore coolant leaks from your water pump. If your water pump is leaking, you should not drive the car. Many of the most common engines today use alloy and aluminum engine parts that can’t tolerate extreme heat and pressure changes. If the water pump stops working, the quick increase in temperature might damage your engine beyond the point of no repair.

In all cases, when your car starts leaking, the faulty component should be replaced as quickly as possible.

Last Words

Your best bet to keep your cooling system in good working condition is regular maintenance. If you do your own mechanic, think about inspecting hoses and other components for leaks at every oil change. Car coolant leaks are significantly less frequent when you maintain your system regularly. If you are not sure how the cooling system work on your car or how to replace a specific component, get your hands on your car’s repair manual beforehand. Always remember that following the manufacturer’s recommended procedure will always save you time and money in the end.


About Derek F

Derek grew up in Southern California and started working on cars when he was a child. He learned from his father and grandfather how to make basic repairs and maintain cars correctly. Derek rebuilt his first engine at 15 years-old, beginning an automotive career that took him to many interesting jobs. Derek has worked as an automotive detailer, managed parts warehouses and auto parts stores, and worked as a mechanic for several years doing brake and suspension work. While pursuing a bachelor’s degree in government journalism, Derek worked for an auto museum where he started to write about cars. Today, Derek uses his expertise gained from many years of practical experience to help educate DIYers and share interesting knowledge about various types of automotive repair and service. Writing about cars helps fund his numerous classic car restoration and customization projects.

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