Hydraulic Lock
A condition where liquid enters an engine cylinder and prevents the piston from completing its compression stroke.
Hydraulic lock, commonly abbreviated as hydrolock, is an engine failure state that occurs when a liquid (typically water, coolant, or fuel) enters an engine cylinder in a volume greater than the combustion chamber volume at Top Dead Center (TDC). Because liquids are virtually incompressible, the piston cannot complete its upward stroke.
If the engine is running or being cranked when a cylinder hydrolocks, the mechanical momentum forces the piston upward against the trapped liquid. This sudden stoppage causes immediate, catastrophic damage. The connecting rod will bend or snap, the piston crown can shatter, the cylinder head may lift or crack, and the engine block can be punctured.
Hydrolock is often caused by driving a vehicle through deep water (where the engine air intake acts as a vacuum cleaner, sucking water into the cylinders), a severe head gasket leak filling a cylinder with coolant, or a failed fuel injector dumping raw fuel. Symptoms include a sudden engine shutdown with a loud metallic clunk, or a starter motor that clicks once and refuses to turn the engine. Hydrolocked engines typically require replacement or a complete rebuild.