Head gasket failure

A failed head gasket lets combustion pressure, coolant and oil find each other across a joint that should keep them apart. The signs are consistent enough to be worth committing to memory, and none of them requires a spanner to find.

The signs

• A mayonnaise-like emulsion under the oil filler cap. Some condensation on a cold engine used for short journeys is normal; persistent, thick emulsion on a warm engine is not.
• Coolant in the oil, or oil in the coolant. Look at the dipstick and into the header tank rather than taking the gauge on trust.
• Coolant loss with no visible leak. If the level falls and nothing drips on the ground, the coolant is going somewhere internal.
• Pressurising of the header tank, with hoses going hard soon after a cold start, or bubbles rising in the coolant with the engine running.

Why the gasket is often the symptom

An aluminium cylinder head that has been run hot enough can warp, and once the head is no longer flat, a fresh gasket alone will not hold for long. The common sequence is an engine that overheated, a head that distorted, and a gasket that gave way. Treating the gasket as the whole problem is how the same failure is bought twice.

Establish the cause before the cure

Before a replacement gasket is fitted, the head needs checking for flatness and the reason for the original overheating needs naming: cooling system, ignition timing, mixture or something else. Skimming a head removes metal and raises the compression ratio, so it should be measured and done deliberately, not assumed. The same discipline applies to the XK six and to the alloy V12, where head work is a larger and more expensive job again.