The head gasket seals the joint between the cylinder head and the block. It has to keep combustion pressure in the cylinder, coolant in the coolant passages, and oil in the oil passages, all at the same time. When it fails, those three get mixed. A cracked or warped head fails in the same way, and the symptoms do not say which of the two has opened the path. Petrol and diesel engines use the same tests. Diesels add a part that imitates the failure: an EGR cooler whose core is a wall between exhaust gas and coolant. Overheating, which both causes gasket failure and follows it, is on engine overheating and coolant faults.
Common petrol engine problems and common diesel engine problems both link here.
What the driver actually sees
The RAC’s description is the common one. Coolant that reaches the combustion chamber leaves as white exhaust, and it usually got there after the engine was overheated. Oil that mixes with coolant leaves a light-brown emulsion. Those signs are a reason to test, not a finished diagnosis.
- White exhaust that clears in the first minute is often condensate in a cold exhaust. White exhaust that continues once the engine is hot, with a sweet smell and a falling coolant level, is coolant being boiled in the cylinder or the exhaust.
- Emulsion on the oil-filler cap after short trips is often water that condensed there and never got hot enough to evaporate. Emulsion on the dipstick, or coolant disappearing into the oil so the level rises, is mixing. A milky cap on its own is a weak piece of evidence.
- The header tank pressurises early, hoses go hard, and coolant is pushed out, sometimes from cold. Combustion gas is heating the coolant from inside. A cap that simply does not hold pressure produces overheating without this gas path. The cap test is on the cooling page.
- A misfire on two neighbouring cylinders is the pattern Engine Australia’s bulletin SB041 associates with a gasket that has failed between those bores. One low cylinder is more often a valve, a ring, or a local crack.
- Overheating that returns after the thermostat, the pump and the radiator have been proved is the loop: gas in the coolant, or coolant leaving the system into a cylinder, puts the temperature back up.
A chemical sealer poured into the coolant is not one of these tests. It hides a leak it has not located, and it does nothing for a cracked head that has not yet been measured.
The tests that localise it
Engine Australia’s SB041 is the compression and leak-down sequence, and it asks for the manufacturer’s method where one exists. The shape of it is:
A dry compression test, battery and starter healthy, plugs or injectors out, throttle open, engine preferably warm, each cylinder cranked through several strokes. Readings should be in specification and, in that bulletin’s rule of thumb, within about 10 percent of each other. That percentage is the bulletin’s screening figure, not a limit stamped on every engine.
If a cylinder is low, a wet test puts a small amount of oil in the bore and repeats. A clear rise points at the rings, the piston or the bore, because the oil temporarily sealed them. No real rise points at valves that do not seal, a valve that cannot close, a worn cam or lifter, or timing. Two adjacent cylinders low, with the others normal, is the bulletin’s gasket-between-bores pattern.
Leak-down uses compressed air at top dead centre on the compression stroke and asks where the air leaves:
- Air at the tailpipe: an exhaust valve is not sealing, or the timing is wrong. Burnt valves live here. Confirm the timing before the head is condemned.
- Air at the intake: an inlet valve is not sealing, or again the timing.
- Air at the oil filler: rings, piston or bore.
- Air from a neighbouring plug or injector hole: the gasket between those cylinders.
- Bubbles in the radiator or header tank: a path into the coolant. The bulletin names a blown gasket, a cracked head or a cracked block. It cannot see which.
A combustion-gas test draws vapour from the header tank through a liquid that changes colour when combustion gas is present. Motor’s July 2011 account of the bromothymol-blue tester describes the colour change from blue to yellow. A positive result proves gas in the coolant. It does not name the cylinder and it does not distinguish a gasket from a crack. Coolant drawn into the tester spoils the liquid. The level has to be low enough that the tool samples gas, and the engine has to be warm enough for the coolant to be circulating. A negative result with the engine cold, or with the leak only opening under load, is not a clean bill of health.
A cooling-system pressure test is the other half. It finds external leaks and, with the plugs or injectors removed, can show coolant arriving in one cylinder. It does not put combustion gas into the coolant, so it will miss a leak that only opens when the cylinder fires.
Diesel and petrol differences
The joint sees higher peak pressure on a diesel, and the gasket construction is chosen for that engine, but the leak paths are the same three. The extra diesel trap is the EGR cooler. A perforated cooler puts coolant into the intake and can put exhaust gas into the cooling system, with white vapour, coolant loss and a positive combustion-gas test. Isolating and pressure-testing the cooler is the step that stops a head being removed for a heat exchanger. Some direct-injection petrol engines have an EGR cooler too. If the engine has one, it is on the list.
Injector sleeves and heater-core plugs in the head are further coolant paths on some diesels. Coolant weeping into the injector well looks like a gasket from the outside of the head and is a sleeve seal. Look there before the head comes off if that engine uses sleeved injectors.
A petrol misfire from coolant in one cylinder will show on the misfire counter and can destroy the catalyst if the car is kept driving. That is a reason to stop, not a clue that the ignition has failed. The ignition checks are on ignition systems and spark plugs, and they come before a head is lifted only when compression has not already localised the fault.
After the head is off
The gasket face tells the truth the tests could only suggest. A fire ring burnt through between bores is a gasket failure between those cylinders. The head face then has to be checked for flatness against the limit for that head, along, across and diagonally. A head that was overheated is often warped, and a new gasket on a warped face fails again. Crack testing, pressure testing the head, and checking the block face are the machine-shop steps. A crack between valve seats or into a coolant passage produces the same bubbles as the gasket did.
The cooling system that caused the overheat, if that was the cause, is part of the repair. A new gasket fitted to an engine whose thermostat is still stuck closed will be asked to seal another overheat. The order is on the cooling page.
Sources
- Engine Australia, service bulletin SB041. Dry and wet compression, the adjacent-cylinder pattern, and where leak-down air leaves the cylinder. The bulletin’s 10 percent figure is a screening guide.
- RAC, engine smoke. White exhaust and emulsion after overheating when coolant and oil cross the gasket.
- Motor Trend, blown head gasket diagnostic. A combustion-gas tester using bromothymol blue, blue to yellow when combustion gas is drawn from the cooling system, and the need to keep coolant out of the test fluid.
- HELLA, exhaust gas recirculation. Context for the cooler that shares the gas-to-coolant boundary on engines fitted with one. The cooler leak itself is distinguished on the EGR fault page by pressure-testing the cooler separately from the gasket.