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How to spot and repair a car radiator leak

How to spot and repair a car radiator leak

Have you got steam or a hissing sound coming from under the bonnet? Is coolant dripping from the underside of your vehicle? If you’ve noticed any of these symptoms, it’s very likely you’ve got a car radiator leak on your hands.

Located at the front of your car, just behind the grill, the radiator is an important component in a vehicle’s cooling system, keeping the engine running at the optimum temperature. But, when your cooling system springs a leak, your engine is at risk of overheating, and left untreated, it could lead to a total engine failure.

In this guide, we’ll guide you through car cooling system troubleshooting , take a look at the causes of a leak, then walk you through a car radiator repair.

What does the car radiator do?

Car engines convert fuel into energy using combustion. Spark plugs inside the engine cylinders ignite the fuel in a controlled explosion, producing the power that propels the vehicle forward.

Naturally, these explosions also create a lot of heat, which means that the engine gets intensely hot while running. If it overheats, it could cause the entire engine to fail. On the other hand, if the temperature falls too low, the fuel economy will drop and Co2 emissions will increase. That’s why a cooling system is needed to regulate the heat and keep the engine at just the right temperature.

A car radiator is the part of the cooling system that prevents the engine from overheating. This cooling system is essentially a series of tubes filled with a liquid coolant, which is circulated through the engine by the coolant pump.

This coolant is then pumped into the radiator, is located at the front of the car behind a grill. The radiator usually consists of two tanks connected by a series of narrow tubes and wires, which increases the surface area of the radiator. The air streaming through this grill as the car moves forward cools the radiator, keeping the engine at the right temperature to run properly.

How can I tell if the radiator is leaking?

It can sometimes be difficult to tell if a problem is in the cooling system or somewhere else in the vehicle. Thankfully, with our car cooling system troubleshooting advice you should be able to tell if you’ve got a car radiator leak on your hands.

Radiator fluid is leaking under the car

If a patch of coolant is leaking underneath your car, it’s a sure sign that you’ve got a leak in the cooling system. But, before you try to locate the source of the leak, you want to be certain that the fluid that’s puddling under your car is definitely engine coolant, as it could also be oil, power steering fluid, or just water.

You should be able to do this by looking closely at the spill. Coolant is usually green or orange in colour, depending on the type of antifreeze in the vehicle, so look closely. Engine oil is normally more yellowish, or even dark brown or black, depending on how long it’s been in the vehicle. Coolant also has a viscous, almost slimy consistency and a slightly sweet smell, while engine oil has a greasy texture and an iridescent sheen.

If your vehicle has power steering, then it could also be power steering fluid: this is usually red or reddish brown in colour. If the spill is colourless it could just be water, especially if you’ve been driving with your air conditioner running, as condensation will often drip down from the compressor.

The car radiator is hissing

When the radiator is hissing, it’s another sign that coolant is leaking, either from the radiator or another component of the cooling system. It could also mean that your radiator fan is malfunctioning, so the water in the radiator isn’t being properly cooled. Or, it could indicate that your radiator cap is leaky, allowing pressure to escape.

The engine is overheating

A radiator leak will stop your cooling system from working properly, meaning the engine will soon start to overheat. If the temperature warning light appears or steam is coming from the under the bonnet, a leak or malfunctioning part is likely to be the culprit.

The radiator is rusty and corroded

Water leaking from the radiator can rust the exterior, so check it carefully for signs of corrosion. It can be easier to see this when the radiator is clean, so wash it thoroughly with a hose (make sure the engine has completely cooled before attempting this). Then, examine it carefully, paying close attention to the underside, as this is where rust is most likely to form as the leaking coolant runs down. If the rust is concentrated in a particular area, this is likely to be the location of the leak.

The coolant level has dropped

Most modern cars have a sealed cooling system, so the level of coolant should remain constant. If it drops, it’s a sign that you’ve got a leak somewhere in the system.

You can easily check this by looking at the see-through radiator water tank under the bonnet while the engine is switched off and cool (check the manual to locate this if you’re unsure). It should be between the minimum and maximum marks on the tank.

If it’s low or nearly empty, you can move on to checking the coolant level in the radiator. To do this, find the radiator cap: this is a pressurised cap just over the bonnet. Cover the cap with a rag to protect yourself from any leaks or steam, and press down and twist to release the cap. The coolant level should be very nearly to the top of the reservoir (some cars have a link marking to indicate the correct level). If it’s low, there may be a leak.

The car runs hot but radiator is cool

When the car is running hot but the radiator is cool, it may not be the radiator itself that is causing the problem: a faulty thermostat could be to blame. The thermostat is a temperature-sensitive valve that controls the flow of coolant through your engine. When the engine gets too hot, the valve opens to allow coolant to flow through the engine, and when the engine is cold, it closes to let it warm up. This regulates the temperature of the vehicle to keep it at the optimum level.

But when the thermostat is faulty, it can become stuck in the closed position, preventing coolant from flowing into the engine, causing it to rapidly heat up. The engine continues to run like this, it could fail, so it’s essential to stop the engine and fix or replace the valve.

To learn more about how to solve this problem, check out our guide on how car thermostats work, and how to change them.

What causes car radiator leaks? Common problems

There can be any number of causes for a radiator leak. Here, we’ve shared some of the most common, to help troubleshoot the issue and work out how to fix it.

A stuck or faulty radiator cap

The radiator cap is a pressurised cap at the top of the radiator. It’s specially designed to let a very small amount of air out of the system, when necessary, to maintain the right level of pressure. If the cap gets jammed, pressure will build up inside the system, eventually causing the radiator hose to crack or break, causing a leak. The cap itself can also start to leak coolant if it becomes damaged: if you notice radiator fluid around the cap and near the radiator, then it’s a likely sign that the cap will need to be replaced.

A leak in the head gasket or engine block

The head gasket is a ringed panel positioned between the cylinder head and engine block. The purpose of the head gasket is to stop the engine oil, coolant, and fuel from leaking into the cylinders. Should the gasket become damaged, coolant fluid could leak out, often along oil and fuel.

Another sign that your gasket is broken is thick clouds of white steam coming out of the exhaust: this is steam, coming from the coolant that has leaked into the hot engine. Left untreated, a blown head gasket could cause your car to overheat, so it’s important to get it repaired as soon as possible.

Worn out or leaky water pump or radiator hose

The water pump drives coolant through the cooling system, but, like any other part, over time it can be subject to wear and tear. It can also be cracked or damaged by impact from sudden knocks and jolts — like hitting a pothole while driving, for instance. A faulty, leaking pump can’t normally be repaired, so you’ll most likely need to replace it.

Damage from road debris or impact

As the radiator is located right at the front of the vehicle, road debris can sometimes be thrown up through the grill, damaging the radiator and causing a leak. It can also become damaged by jolts, bumps, or collisions. You can usually find the damage by inspecting the radiator carefully. A damaged radiator will often need to be fully replaced.

Cold weather

Icy weather is another common culprit behind damaged radiators. Water expands when it freezes, and if the coolant inside the engine is allowed to freeze in the cold, it can crack the radiator or burst the radiator hose.

This is why it’s so important to add antifreeze — usually ethylene glycol — to the coolant, as this lowers the freezing point. The recommend level of antifreeze for most vehicles is a 50:50 ratio — in other words, equal parts antifreeze and water. While antifreeze is designed to last for at least a few years, engine coolant degrades over time, and it can also gradually boil off. So, it’s important to top it up regularly — preferably before winter sets in. You should also flush the radiator every few years to clean it out and remove any contaminants that could stop it from working properly.

How to stop and fix a car radiator leak

Before you can fix the problem, you’ll need to identify the exact location of the leak — remember, it may not be the radiator itself, but another part of the cooling system. If you’ve ruled out the possibility of a leak elsewhere in the system, then you’ll need to either

Cooling system leaks can be incredibly difficult to spot. If you still can’t find the leak after a thorough visual inspection, take the car to a mechanic, who can use specialist pressure-reading tools to identify which part of the cooling system is the source of the leak.

Using a repair sealant system

If the leak is very small (barely visible to the naked eye) you may also be able use a radiator repair sealant additive to repair pinpoint leaks. These products are added to your cooling system via the reservoir and will expand to fill and seal the crack, and, as a result, are usually more of a temporary fix. But, you should bear in mind they won’t do anything for severe leaks and damage. However, they can certainly be useful if you need to get your car up and running so you can take it to a garage.

Flush and replace the radiator

When the radiator is severely damaged or cracked, you’ll need to fully replace it. This can be quite a complicated process, so if you’re not confident about doing it yourself, we’d advise enlisting the help of a qualified mechanic.

If you’re replacing the radiator yourself, remember that you’ll need to drain all the fluid out of the system completely first: you can learn more about this in our guides on how to flush a radiator.

A car radiator leak can reduce the effectiveness of your entire cooling system. If you don’t get it repaired, it could cause your entire engine to overheat — and that can cost a pretty penny to fix. This guide should help to work out whether it’s your radiator or another part of the cooling system that is leaking, so you can identify the source of the problem and get it sorted as soon as possible.

At GSF Car Parts, have a huge range of cooling system parts and general tools, so take a look if you need to stock up before you tackle to job. To find more handy repair tips, take a look at our car advice centre and knowledge hub.