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Car coolant leaks: the complete guide

Puddle of coolant under car

Why car coolant level drops

There are a couple of reasons why your car coolant is low. First, you recently renewed the coolant and the air locks in the system have now cleared and the coolant needs to be topped up to the correct level. Second, there's a leak. Car coolant is contained within a sealed system so, if it's in good condition, the coolant level should not 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 when it cools).

But don't panic expecting the worst. Many leaks are easily fixed – just be prepared to spend some time with your head under the bonnet as some leaks can be tricky to locate.

Where does car coolant go when it leaks?

Car 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 or the engine block and can see a pool of coolant under the car then it's an external leak. If not, it's internal, which means the fluid is leaking into the engine.

What can cause external car coolant leaks?

Header tank cap

Header/expansion 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. 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.

Coolant pump seal

Coolant pump

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 coolant hose

Split car 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 coloured 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, as over time they can 'bite' into the rubber and cause splits.

Radiator leak

Radiator

Stone chips and age are a radiator's worst enemy. The trouble is, if the 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 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

Your car's coolant also keeps you nice and warm in the winter via the heater matrix. This is usually mounted behind the dashboard and occasionally leaks. The maxtrix's location makes it difficult to spot a leak promptly, but if you've recently noticed both coolant loss and your car suffering from internal condensation 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.

Other causes of 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. 

What’s a car 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 can cause internal car coolant leaks?

If the car coolant level is going down, and there is no external leak evident, then it's 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.

Possible signs of head gasket failure include:

  1. Coolant mixed with the oil (turning it milky if there is a severe leak).
  2. The underside of the filler cap 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) coming out of the exhaust, even when the engine is warm.
  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 pressurises 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.