Overheating or heavy white smoke? Switch off
If the temperature gauge is high, the temperature light is on or thick white smoke is coming from the exhaust, pull over somewhere safe and turn the engine off. Don't drive on to get home, and don't open the radiator or coolant cap on a hot engine. If the car has stopped in traffic, get everyone clear of it and call roadside assistance if you have it. If anyone is hurt or in danger, call 000.
What does a head gasket do?
An engine is two big pieces bolted together: the block, where the pistons run, and the cylinder head on top. The head gasket is the thin seal between them. It has one of the hardest jobs in the car, because it keeps burning gases inside the cylinders and keeps coolant and oil out of them, side by side, through thousands of hot and cold cycles.
When it fails, it can leak in a few directions at once. That's why the symptoms look so mixed, and why two cars with the same fault can act completely differently.
Head gasket symptoms, and what else causes them
| Why the gasket can cause it | What else can look the same | |
|---|---|---|
| Engine overheating | Combustion gas pushes into the cooling system and forms air pockets. | A coolant leak, thermostat, water pump or blocked radiator. |
| Coolant going missing, no puddle | It's being burnt in the engine or mixing with the oil. | A small leak that only opens up when the engine is hot. |
| Milky, creamy oil on the dipstick | Coolant has mixed with the oil, and it can look like a milkshake. | A little condensation under the oil cap, usually from short trips. |
| Thick white exhaust smoke | Coolant is burning in a cylinder. | Steam on a cold morning, or a diesel fuel problem. |
| Bubbles or foam in the coolant bottle | Exhaust gas is being forced into the coolant. | Air in the system after a recent coolant change. |
| Rough running, loss of power | A cylinder has lost compression. | A misfire, from ignition or fuel faults. |
Why does it blow?
Overheating is the one to watch. An engine that's run hot can distort the head, which lets the gasket leak, and the leak then makes it run hotter. Gases in the cooling system can also form air pockets that trap the thermostat closed, and that causes more overheating.
Age and severe engine knock can also do it. That's why fixing the cooling fault matters as much as the gasket. If the reason it overheated is still there, the new gasket is on borrowed time.
RACQ warns that overheating damage may not show for days, weeks or even months. So if your car boiled a while ago and now it's misbehaving, tell the mechanic about that first.
What to do now
Stop driving it
A short trip can feel fine, but each overheat can warp the head further. Ask for a mechanic who'll come to the car, or arrange a tow.
Check the oil on a cold engine
Pull the dipstick once it's cool. Creamy, coffee-coloured oil is the clear warning. Take a photo.
Look at the coolant bottle
With the engine cold and the cap left on, check the level and whether it looks oily or brown.
Note what you've topped up
How much coolant, how often, and any time the gauge went high. It saves the mechanic testing for what you already know.
How will the mechanic confirm it?
Don't accept a blown head gasket on symptoms alone. The usual tests are a cooling system pressure test, a chemical test for exhaust gas in the coolant, and a compression or leak-down test on each cylinder. Together they show whether it's the gasket, a cracked head or something much simpler.
If it is the gasket, the head has to come off. A good written estimate covers checking the head for warping and cracks, machining it if needed, new bolts where the maker calls for them, fresh oil and coolant, and fixing whatever caused the overheat. Timing components may have to come off too, so ask whether it's sensible to replace them while they're out. RACQ notes the full extent of damage may only be clear once the engine is dismantled, so ask how they'll handle it if they find more.
This is a big engine job. Ask how often the mechanic does head gaskets on your kind of engine, since aluminium engines and diesels each need care.
What you can do, and what's the mechanic's job
You can
- Stop driving it and let the engine go fully cold.
- Check the dipstick and coolant bottle, with the cap left on.
- Write down when the car overheated and what you topped up.
Leave to the mechanic
- Pressure, chemical and compression testing.
- Taking the head off, and checking it for warping and cracks.
- Finding and fixing what caused the overheat.
If it were our car
We'd stop driving it and get it tested before anything else. Once it's confirmed, we'd weigh the repair against what the car is worth. The NRMA's advice is to compare the bill with the market value, think about the car's age and kilometres, and get three quotes. A well-kept car with years left in it is usually worth repairing. An older one with other problems may not be, and a straight answer on that is part of a mechanic's job.