Head Gasket — The Critical Seal

The head gasket sits between the cylinder head and the engine block. It is a single gasket that must simultaneously seal combustion pressure at over 1,000 PSI, coolant passages at 15 PSI, and oil passages — all in a joint that cycles from cold to over 200 degrees Fahrenheit and back hundreds of thousands of times. It is one of the hardest-working gaskets in the entire vehicle.

A blown head gasket can fail in several ways depending on where the breach occurs. Combustion gases leak into the cooling system — you see bubbles in the coolant reservoir or radiator with the cap off and the engine running. The cooling system pressurizes rapidly. Coolant leaks into the combustion chamber — white sweet-smelling smoke from the exhaust that does not go away after warmup. Coolant level drops with no visible external leak. Oil and coolant mix — a milky tan substance appears on the oil cap or dipstick. Cylinders leak into each other — two adjacent cylinders show low compression.

Chemical block test — the most definitive shop test. A chemical fluid in a test tool changes color when exposed to combustion gases. Place the tester in the radiator neck or reservoir opening with the engine running and the cap off. If the fluid changes color, combustion gases are entering the cooling system through the head gasket. Compression test — two adjacent cylinders with low compression that improves when tested together points to a gasket breach between those cylinders. Pressure testing the cooling system — if the system loses pressure with no visible external leak, suspect an internal leak through the head gasket into the combustion chamber or oil passages.