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What causes a coolant leak?

Coolant leak under a car

Why is the coolant level low?

There are a couple of reasons why your car's coolant level might be low, and a leak won't always be to blame.

You might have just changed the coolant and the air locks in the system have now cleared and the coolant needs to be topped up to the correct level.

If you haven't just renewed the coolant, there's a good chance there is a leak, either from the radiator or somewhere else. Car coolant is contained within a sealed system, so if it's in good condition the coolant level shouldn't change if it's checked when the engine is cold. The level will rise when the engine is warm because water expands when it's hot, but it'll fall back to the 'cold' level when it cools.

If the level is dropping, don't panic. Coolant leak repair is usually straightforward – just be prepared to spend some time with your head under the bonnet because some leaks can be tricky to locate.

What’s the difference between an internal coolant leak and an external leak?

A car’s coolant doesn't just evaporate into thin air when it leaks (well, not all of it, anyway).

There are two kinds of coolant leaks: internal and external. If you can locate a drip on a hose, the engine block or can see a radiator leak, and there's a pool of coolant under the car, then it's an external leak.

If there are no puddles beneath the vehicle, yet the coolant level is dropping, you have an internal leak, which means the coolant is leaking into the engine.

What causes external car coolant leaks?

Expansion tank tap

Broken header tank cap or radiator cap

The car coolant header tank is equipped with a pressure release valve in the filler cap, intended to release coolant harmlessly in the event that the pressure within the system is too great.

Sometimes the spring/valve within the cap can weaken and it can release coolant prematurely.

This will reveal itself with coolant residue around the header tank (shown here). If the coolant system is otherwise in good order a new cap could be the solution.

The same is true for cars without a header tank, because the radiator cap performs the same function, and can fail over time.

Water pump

Leaky coolant pump seal

The coolant pump has a seal which prevents coolant from leaking past the bearing. If this seal fails then coolant can seep out where the coolant pump pulley shaft enters the pump housing.

There could also be a leak between the pump and engine block if the pump's gasket has failed.

It may be evident with traces of car coolant around the engine bay and even on the underside of the bonnet, because the coolant pump pulley can fling the coolant everywhere!

Split hose

Split coolant hose

Even the tiniest pinprick in a hose can cause a coolant leak, and sometimes this leak is only evident when the engine is hot.

There may not be enough pressure in the system with the car just ticking over to make the leak apparent. But there will be telltale signs of coolant around where the leak is because it often leaves behind a colored residue.

Larger splits will be obvious if you have a good look around the engine. Also, inspect where the metal clips secure the hoses at each end – over time they can 'bite' into the rubber and cause splits.

Radiator leak

Stop that radiator leak!

Stone chips and age are a radiator's worst enemy, and a holed rad usually leaks water like there's no tomorrow. The trouble is, if the radiator leak is small, the car coolant could evaporate before it's had a chance to pool on the floor and give away its location.

So if you can't find any apparent radiator leak, look for small puffs of steam emanating from the radiator when the car is hot, or any discoloured areas on the radiator's fins. There will also be more evidence than the usual smell of coolant in the engine bay, so get sniffing!

Heater matrix

Heater matrix leak

Your car's coolant also keeps you nice and warm in the winter via the heater matrix. This is usually mounted deep behind the dashboard and occasionally leaks.

The heater matrix's location makes it difficult to spot a leak promptly, but if you've recently noticed both coolant loss and condensation on the inside of the windows, it could point to the matrix being at fault.

Also, check the carpet under the dashboard for any signs of moisture – and use your nose. If it's leaking, the interior will almost certainly smell 'coolanty'. Your heater may also not work as effectively, if at all.

What else can cause a coolant leak?

Pretty much anything that either carries water or prevents water from exiting the engine can be at fault, whether that's a split header tank, a leaking thermostat gasket, or a hose not properly secured.

A thorough inspection of the engine bay should take all areas into consideration. 

Coolant pressure tester

There are also situations where there is a leak, but the cause is not apparent – a very small leak in the radiator for example could evaporate away before it's noticed, as mentioned above. In these instances, it's worth getting a garage to carry out a coolant pressure test.

This pressurises the system to replicate the effect of a running engine. As a result, any coolant leak will be more obvious.

Blown head gasket

What causes internal car coolant leaks?

If the coolant level is dropping and there is no external leak evident, then the coolant is probably leaking internally, into the engine.

If the car has recently overheated then this could have caused the head gasket to fail. If it has, it could be leaking coolant into the combustion chambers.

Clues of head gasket failure:

  1. Coolant mixed with the oil (turning it milky if there is a severe leak).
  2. The underside of the filler cap is covered in a mayonnaise-like substance – although this is more likely to be caused by condensation if the car is only ever used on short journeys.
  3. White smoke (steam) comes out of the exhaust, even when the engine is up to operating temperature.
  4. Car overheating.
  5. Lumpy running, rough idle, poor starting.
  6. Coolant bubbling or 'froth' in the header tank.
  7. Water dripping from the exhaust manifold.

A garage can use a gas analyser to detect a head gasket leak. It tests for hydrocarbons (unburnt fuel) in the coolant system. If there is any present the gasket is blown.

There is also the option of a leak-down test which pressurizes the engine (through a spark plug hole) and monitors the pressure loss.

If the head gasket has failed, there is only one solution, and that's to strip the engine down and replace the gasket. Your Haynes Manual will show you how to do this.