Head gasket failure: signs to look for at a viewing

A failing head gasket usually gives itself away through coolant: the level keeps dropping with no leak you can see, the engine runs hot, or white smoke keeps coming from the exhaust after the engine is warm. None of these signs proves it on its own. Together, or with a seller who has just topped up the coolant, they are a reason to ask for a proper test or to walk away. The MOT will not catch it, so the checks at the viewing matter.

What the head gasket does

An engine is built in two main parts: the block at the bottom and the cylinder head on top. The head gasket is the seal between them. It keeps three things apart: the combustion gases in the cylinders, the coolant that keeps the engine at the right temperature, and the oil that lubricates it.

When the gasket fails, those can mix. Hot gases can get into the coolant, coolant can get into the cylinders or the oil, and oil can get into the coolant. Each mix causes its own symptoms.

A common cause is overheating. An engine that has run very hot, for example after a coolant leak or a failed water pump, can warp the cylinder head and damage the gasket. So a head gasket problem often comes with a cooling system story.

The signs to look for

Coolant level dropping

Check the coolant level in the expansion tank against the minimum and maximum marks. Ask the seller how often they top it up. A car that keeps needing coolant, with no puddle and no damp hoses, may be losing it inside the engine.

Safety first: never open the coolant cap on a hot engine. The system is under pressure and hot coolant can spray out. Check it cold.

A fresh top-up before you arrive

Look for signs the coolant has just been filled: a level exactly at the maximum on a car that has been driven, drips around the filler, or a fresh container of coolant in the boot. Ask directly whether it has been topped up recently and why.

Mayonnaise under the oil cap

A creamy, light brown sludge under the oil filler cap can mean coolant is mixing with the oil. But there is an important caveat: cars used mostly for short trips can build up the same sludge from condensation, because the engine never gets hot enough to drive the moisture off. So treat it as a reason to look further, not as proof. Check the dipstick too. Milky oil on the dipstick is more worrying than a little residue under the cap.

White exhaust smoke after warm-up

On a cold or damp day, most cars puff white vapour from the exhaust when first started. That is water vapour and it clears as the exhaust warms up. White smoke that keeps coming once the engine is fully warm, especially with a sweet smell, can mean coolant is burning in the cylinders.

Overheating

Watch the temperature gauge on the test drive. It should rise to its normal position and stay there. A needle that keeps climbing, or a cooling fan that runs constantly, is a warning. The Highway Code says that if warning lights stay on or come on while you are driving, you should stop and investigate, because you could have a serious fault.

Bubbles in the expansion tank

With the engine running and the cap left on, look at the coolant through the side of the expansion tank. A steady stream of bubbles can mean combustion gas is being pushed into the cooling system.

Hard coolant hoses soon after a cold start

The cooling system builds pressure as it warms up. If the top hoses go hard very quickly after a cold start, before the engine is properly warm, gas may be leaking into the system. Be careful near moving parts and hot surfaces.

Rough running or misfiring

If coolant gets into a cylinder, the engine may run roughly, especially just after starting.

Why a cold start matters

Many of these signs are easiest to see from cold. Steam that clears normally, hoses that stay soft and a temperature gauge that rises slowly are all reassuring. A seller who warms the car up before you arrive can hide the start-up smoke and the hose pressure.

Ask the seller not to start the car before you arrive. When you get there, feel the bonnet. If it is warm, ask why. Then check the coolant level cold, start the engine yourself and watch the exhaust and the expansion tank. Our test drive checklist covers the rest of the drive.

What the MOT does and does not show

The MOT will not tell you the gasket is healthy. GOV.UK says the test does not cover the condition of the engine. The DVSA inspection manual also says that a leak of engine coolant is not a reason for failure, so a car can lose coolant and still pass.

The test does check emissions and smoke, but the rules differ by fuel. On a petrol car, the visual smoke check looks for dense blue or clearly visible black smoke, so white smoke is not one of the listed defects. On a diesel, the manual makes smoke or vapour of any colour that is likely to obscure other road users' vision a dangerous defect, and a diesel with a particulate filter fails for visible smoke of any colour. A car with an early gasket problem can easily pass. Our guide to what the MOT does not check covers the gaps.

Tests a mechanic can do

If you see warning signs but still like the car, a garage can test for a head gasket problem. Common tests include:

• A combustion leak test, sometimes called a block test, which checks the coolant for exhaust gases.
• A compression test, which measures the pressure each cylinder can hold.
• A leak-down test, which shows where pressure is escaping.
• A cooling system pressure test, which looks for leaks.

Ask the seller whether you can have one done before you buy. A seller who refuses is giving you information.

When to walk away

Walk away, or at least pause, if you see:

• white smoke that does not clear once the engine is warm;
• a temperature gauge that climbs on the test drive;
• bubbles in the expansion tank with the engine running;
• milky oil on the dipstick;
• a fresh coolant top-up the seller cannot explain.

A head gasket repair involves taking the top of the engine apart, and the head may need machining if it has warped. It is a big job. If you are offered the car "with a head gasket issue" at a low price, see the car and ask a mechanic before you agree to anything.

Frequently asked questions

What are the first signs of head gasket failure?

Usually coolant loss with no visible leak, the engine running hotter than normal, or white exhaust smoke that keeps going once the engine is warm.

Does mayonnaise under the oil cap mean the head gasket has gone?

Not always. Short trips can cause the same sludge through condensation. Check the dipstick for milky oil and look for other signs before drawing conclusions.

Will a blown head gasket fail an MOT?

Not necessarily. The MOT does not check the condition of the engine, and a coolant leak is not a reason for failure. Smoke can fail the emissions part, but on a petrol car the visual check is for dense blue or black smoke, not white.

How can I test for a head gasket problem before buying?

Ask a garage for a combustion leak test or a compression test. Both are common and give you more than a visual check can.