Signs of a Blown Head Gasket

What the head gasket is sealing

A cylinder head gasket seals three things at once between the head and the block: combustion pressure in each cylinder, oil in the galleries that feed the valvetrain, and coolant in the passages that cross the joint. When it fails, those systems mix or the combustion gas escapes. The symptom depends on which part of the gasket let go.

One sign is not a diagnosis. White vapor on a cold morning, a sweet smell, or a milk-colored film under the oil cap all happen on healthy engines. The patterns below are the ones that keep showing up after the engine is hot, and the checks that separate a head gasket from a radiator cap, an intake gasket, or a cracked head.

Coolant disappearing into a cylinder

Coolant leaving the overflow with no puddle under the car is the classic combustion-side failure. The engine may run, then overheat once the level drops. Exhaust stays white and smells sweet after the engine is fully warm, not just for the first minute of a cold start. A spark plug in the affected cylinder can look steam-cleaned, and that cylinder misfires under load.

A combustion-gas tester (the blue fluid block tester) on the radiator or pressurized overflow will change color if exhaust is being pushed into the coolant. Bubbles in the radiator with the cap off and the engine running point the same way, once you have ruled out a low level that is simply pulling air. A cooling-system pressure test that loses pressure with no external leak means the coolant is going into a cylinder, into the oil, or out through a crack you have not found yet.

• Persistent white exhaust smoke after warmup, plus unexplained coolant loss.
• Overheating that comes back as soon as the coolant level drops.
• A sweet smell from the tailpipe or the heater, not only from a cold engine.
• Combustion gas in the coolant on a block test.

Oil and coolant mixing

A breach between an oil gallery and a coolant passage puts coolant in the oil or oil in the coolant. Coolant in the oil turns the dipstick milky or tan and raises the level on the stick. Oil in the coolant leaves a brown film in the radiator or reservoir. Either one is a reason to stop driving: coolant does not lubricate bearings, and oil-contaminated coolant stops carrying heat.

The underside of the oil cap gets a light tan foam on engines that only take short trips, because moisture in the crankcase never boils off. Check the dipstick and the coolant, not the cap alone. If the dipstick oil is clear and at the right level, the cap film is condensation. If the stick is milky, treat it as coolant in the oil until a lab or a pan drop says otherwise.

Compression leaking between cylinders

The gasket can fail between two combustion rings and never touch a coolant hole. The engine misfires, idles rough, and has two adjacent cylinders with low compression. Coolant level and oil stay normal. A leak-down test on one of those cylinders sends air out the spark-plug hole of the neighbor. That pattern is a head gasket, a cracked head, or a burnt valve area between the cylinders, and the head has to come off to see which.

A single dead cylinder with air escaping from the intake or the tailpipe is a valve, not the gasket. Air escaping from the radiator neck during leak-down is combustion leaking into a coolant passage, which is the gasket or a crack in the head or block.

External seepage is a weaker clue

Coolant or oil wetting the outside of the head-to-block joint can be the gasket, especially at a corner or around a coolant port. It can also be a leaking intake end seal, a timing cover, a freeze plug, a heater hose, or a warped thermostat housing that only appears to come from the gasket. Clean the area, drive until the engine is hot, and see where the fresh fluid starts. An external seep with a full coolant bottle and clean oil is not, by itself, a blown gasket.

When to stop driving

Continued driving with coolant loss overheats an aluminum head and warps it. The new gasket will not seal that head until it is measured and, if it is out of spec, machined. If the engine has already overheated into the red, or the oil is milky, do not keep topping up the coolant and driving. Towing is cheaper than a head and a set of bearings.

A Fel-Pro head gasket or head set is the replacement part once the joint is open and the head checks flat. It is not a way to confirm the failure from the driveway. Match the part to the year, make, model, and engine on this site, then read what else has to be replaced with the gasket before the head goes back on.