What are the signs of a head gasket failure?

The four symptoms that actually indicate a failed head gasket, how to tell each from the harmless thing it is confused with, and why continuing to drive is the expensive mistake.

Difficulty: beginner
Time: 15 min
Read: 5 min
Safety: warning

Short answer

The signals worth acting on are coolant disappearing with no puddle anywhere, thick white steam from the exhaust that does not clear as the engine warms, creamy mayonnaise-like residue on the dipstick, and the expansion tank bubbling or overflowing while the engine runs. One of those on its own can have an innocent explanation. Two together is a head gasket until proved otherwise, and a garage can confirm it with a chemical test on the coolant in minutes.

The head gasket seals the joint between the cylinder head and the engine block. It has to hold back combustion pressure, coolant and oil at the same time, across a joint that heats and cools by hundreds of degrees, and it does that for the life of most engines. When it fails, the three fluids that were meant to stay apart start mixing, and which way they mix determines what you see. Combustion gas pushed into the coolant pressurises the system and pushes coolant out. Coolant drawn into a cylinder produces white steam and a sweet smell. Coolant into the oil produces the emulsion people call mayonnaise. Understanding which direction the leak is going explains every symptom in this guide.

Safety

Never open a radiator cap or expansion tank on a warm engine — the system is pressurised well above the boiling point of the coolant and it will spray scalding liquid. Wait until the engine is cold to the touch, which takes far longer than people expect. If you suspect a head gasket, stop driving: combustion pressure in the cooling system, or coolant in the oil, destroys an engine quickly and cheap sealants make a professional repair harder.

Step by step

Establish whether coolant is genuinely disappearing.Mark the level on the expansion tank with a pen when the engine is cold, and check it cold again after a few days of normal use. A system that is healthy does not consume coolant. A level that falls with no puddle, no wet hoses and no drips is the strongest single indicator there is.
Look at the exhaust properly.Every car produces white vapour on a cold morning, and it clears within a minute or two as the exhaust warms. Thick white smoke that persists once the car is hot, smells faintly sweet and leaves the tailpipe wet is coolant being burnt. That is not the same thing at all.
Check the oil filler cap and the dipstick.A light creamy deposit on the underside of the cap alone, on a car that does short cold journeys, is usually condensation and is common in winter. The same emulsion on the dipstick, or oil that has gone milky brown throughout, means coolant is getting into the oil and that is serious.
Watch the expansion tank with the engine running from cold.Take the cap off only when the engine is stone cold, start it, and look at the coolant. A steady stream of bubbles rising through it, or the level rising and overflowing as the engine warms, is combustion gas being forced into the cooling system. Stand clear and put the cap back on before revving anything.
Notice how the heater behaves.A heater that blows cold while the temperature gauge is normal often means air in the system, and the air got there somehow. Combined with disappearing coolant, that points the same way.
Take overheating seriously as both cause and effect.Most head gaskets fail because the engine overheated first — a failed fan, a stuck thermostat, a leaking hose. Once the gasket has gone, the car overheats more readily, which makes it worse. This is why one episode of severe overheating must be investigated rather than shrugged off.
Ask a garage for a combustion gas test on the coolant.It uses a fluid that changes colour in the presence of exhaust gases in the coolant, takes a few minutes, and is inexpensive. It is far better evidence than a list of symptoms, and it also rules the gasket out — which matters, because plenty of these symptoms have cheaper causes.
Stop driving it while you decide.Every additional heat cycle enlarges the failure, and coolant in the oil strips the bearings' protection. The difference between a gasket job and a replacement engine is often a few hundred miles of denial.

Common mistakes

Pouring in a head gasket sealant and carrying on — At best it buys a few weeks. At worst it blocks the radiator and heater matrix as well, so the eventual proper repair costs more. It also makes the car unsellable in good conscience.
Topping the coolant up every week and treating that as maintenance — It normalises a fault that is destroying the engine. A cooling system is sealed; if it needs topping up, something is wrong.

If it doesn't work

Coolant low, but there is a visible drip or a wet hose

Cause: An external leak, not a head gasket — Fix: Far better news and far cheaper. Hoses, jubilee clips, a radiator seam, the water pump and the expansion tank cap are all ordinary failures.

Mayonnaise only on the oil filler cap, oil clean on the dipstick

Cause: Condensation from repeated short cold journeys — Fix: Common on cars doing two-mile trips in winter. Take a longer run occasionally so the engine gets properly hot and boils the moisture off, and check the dipstick rather than the cap.

White smoke that clears within a minute or two

Cause: Ordinary condensation in a cold exhaust — Fix: Normal, especially on a frosty morning. Judge by whether it persists once the car is fully warm.

Overheating in traffic only, coolant level fine

Cause: Cooling fan or its relay — Fix: A classic and comparatively cheap fault. At speed the airflow does the work; stationary the fan has to.

Engine runs rough on cold start then clears

Cause: Can be an early gasket leak into one cylinder, or ordinary ignition wear — Fix: Have the codes read. A persistent misfire on one cylinder combined with coolant loss is a strong pointer; on its own it is more likely a plug or coil.

Questions people ask

Is a head gasket always worth repairing?

It depends entirely on what the car is worth and whether the head has warped. The gasket itself is cheap; the labour is substantial and the head usually needs skimming and pressure testing. On a car worth a few thousand it is often sensible; on an older one it frequently is not, and the catalogue's guide on whether to repair a car covers that arithmetic.

Can a head gasket fail without overheating?

Yes, though it is less common. Age, a poor previous repair, incorrect bolt torque, or a small localised failure between two cylinders can all happen without the gauge ever moving. It is why the coolant-level test matters more than the temperature gauge.

How do I tell a head gasket from a cracked head?

You often cannot from the driver's seat — the symptoms overlap almost completely. The distinction is made when the head comes off and is pressure tested, which is why any quote for a gasket should be conditional on what they find.