Head gasket
Definition
The head gasket seals the joint between cylinder head and block, separating combustion pressure, coolant passages and oil galleries; modern gaskets are multi-layer steel with elastomer beads. It fails from overheating (head warps and relaxes clamping), detonation, corrosion from old coolant, and improper torque at a previous repair. Symptoms depend on where it breaches: combustion gas into coolant (bubbling in the expansion tank, pressurised hoses, overheating), coolant into a cylinder (white sweet-smelling exhaust, misfire on cold start, coolant loss without external leaks), or oil-coolant mixing (mayonnaise sludge). Confirm with a block tester (chemical CO2 check on the coolant), cooling system pressure test, cylinder leak-down or an exhaust gas analyser probe over the expansion tank. Codes: P0300-P0304 (misfire on the wet cylinder), P0128/P0217 (coolant temperature issues), P0171 if steam is read as lean; a compression test shows two adjacent low cylinders when the gasket leaks between bores.