Hydrostatic lock, hydraulic lock or hydrolock occurs when liquids, typically water, enter an engine cylinder. This can occur from a coolant, oil or fuel leak, but the chief cause is drawing water into the engine through the air induction system (airbox & filter, ducting, throttle body or carburetor, intake manifold). Internal combustion engines (spark or compression ignition) operating on a two-stroke or four-stroke cycle must employ a compression stroke to compress the charge (usually an air/fuel mixture). Liquids are incompressible; the presence of a liquid in the engine cylinder during the compression stroke generates destructively high cylinder pressures.
Abnormally high cylinder pressures can bend and break pistons, piston pins, connecting rods, crankshafts and ruin bearings and can crack or break cylinder heads and engine blocks. Small amounts of liquids may pass through an engine cycle without damage, but volumes exceeding 40cc (1.4 fluid ounces, <3 tablespoons) will cause many engines to develop cylinder pressures well in excess of 1000psi. A larger volume of water, up to the combustion chamber volume (usually 60cc to 100cc), will generate increasingly high cylinder pressure during the completion of the compression stroke. Volumes of water which exceed the combustion chamber volume will "stop" a running engine through true hydrostatic lock. Something expensive always bends or breaks when this happens.
Hydrolock may occur while the engine is running, the work of the compression stroke being supplied by engine's rotational inertia. Or a liquid may leak into the cylinder while the engine is being stored; the work of the compression stroke will be supplied by the starter motor.
Hydrolock is not a new problem, but it only affected certain applications. Older American made cars, particularly with V-configuration engines, often employed an air intake location which was high in the engine compartment. Because of the reduced tendency of these older American cars to hydrolock, it has not been in the forefront of design consideration and is not a household term.
Most newer, fuel injected cars have the air intake located low in the engine compartment. The objective of this low air intake is to draw cool air into the engine. Unfortunately, when driving through sufficiently deep standing or splashing water, engine vacuum from the intake stroke will suck water into the engine, particularly if the intake is submerged.
The cost to repair hydrolock damage begins at about $1000.00, and only goes sky high from there. Repair bills in excess of $35,000 have been reported in high-end passenger cars. Racing engines can cost twice that amount. Many new cars have been recognized as having poor designs to prevent hydrolock. Even some Four-Wheel-Drive pickup trucks and SUV's have been identified as having particularly high incidence rates of hydrolock. Manufacturers' warranties do not cover hydrolock engine damage, stating that the cause of operator error. One SUV manufacturer states the maximum vehicle speed through standing water to be 5 mph; hydrolock occurring at speeds in excess of 5 mph is judged to be operator error. The repair may be covered by a vehicle's Collision/Comprehensive insurance.