What Is a Head Gasket and How Can You Tell if It's Blown?

Discover what this engine component is and a few signs that might indicate when it has gone bad.

Detail view of a smoking engineShutterstock


Blown head gasket is one of the more alarming conclusions a mechanic can make when diagnosing an engine problem. While a head gasket is a relatively cheap part to buy, it's time-consuming — meaning expensive — to replace.

A Head Gasket Creates a Seal Between the Engine Block and Head

Regardless of what you drive, your vehicle's engine consists of hundreds of different parts. One of the more critical is the engine's head gasket. It creates a seal between the engine block, which houses components such as the pistons, and the cylinder head, where the valvetrain is typically located. The head gasket's primary role is to contain compression inside the combustion chambers. It also allows coolant to flow between the block and the head without entering the combustion chambers and keeps engine oil out of the cooling system.

Most modern cars use a head gasket made with multiple layers of steel, including outer layers coated in polymers capable of withstanding high temperatures. Engines with a flat-cylinder layout, such as the flat-four found in Subaru models, and engines with a V-shaped layout, such as the V6 and V8 units that power many performance cars, pickups, and SUVs, have head gaskets on each cylinder bank.

A Head Gasket Can Last 200,000 Miles or Longer

With three tasks to juggle, the head gasket stands out as one of the most important parts of an engine. As it's relatively costly to replace, most carmakers design head gaskets that last for about 200,000 miles.

Factors such as how you drive and maintain your car can affect that figure, and it's not uncommon for them to last much longer. However, if an engine is overheated or part of the cooling system fails, a head gasket can fail much sooner.

A Blown Head Gasket Can Manifest Itself in Several Ways

While a "blown head gasket" error message won't appear in your car's instrument cluster, a failure can manifest in several ways. One of the most common symptoms is an engine that runs poorly. It might have a rough idle, deliver poor acceleration, overheat, or exhibit a combination of all three issues. This happens because the engine makes less power as it loses compression.

When taking a car to a repair shop to diagnose this issue, it's common for a mechanic to use a block testing fluid that tests for the presence of hydrocarbons — aka unburnt fuel — in the coolant. If there's fuel in the coolant, a blown gasket is a likely root cause.

In a worst-case scenario, the head gasket fails, and your engine starts burning the coolant that enters the combustion chamber. When this happens, the tailpipes will emit thick white smoke. Unlike condensation on a cold morning, this smoke won't disappear once the engine reaches operating temperature. Eventually, the coolant level will drop as the engine burns it off, increasing the risk of overheating and causing expensive damage.

Engine oil that enters the cooling system via a leaking or blown head gasket gradually blends with the coolant and turns into a milky-looking light-brown substance with a consistency resembling mayonnaise. This becomes visible under the engine oil cap, on the dipstick, and in the coolant tank.

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Ronan Glon
Ronan Glon is an American journalist and automotive historian based in France. He enjoys working on old cars and spending time outdoors seeking out his next project car.