A hydrostatic lock, or hydrolock, occurs when a liquid, usually water, gets into an engine cylinder and prevents the piston from working. The amount of damage caused by a hydrolock varies depending on the circumstances under which it occurs. Some hydrolocks cause no lasting damage and can be resolved quickly; others will effectively destroy the engine.
Liquid in an engine cylinder is usually a result of flooding. Drivers who try to drive through areas of deep water may find that water floods in through their air intake. Cars with cold air intakes, which are usually lower, are especially vulnerable to this problem. Driving quickly through a patch of water can create a bow wave, flooding the intake even if it is above water level. In rarer cases, hydrolock can occur when fluid, such as coolant, leaks within the engine itself.
Hydrolock at low speeds
At low speeds, hydrolock is not always very damaging to the engine. If a piston in an engine that is idling or moving at very low speed fails to complete its movement in the cylinder, the engine will usually simply stop. In the best-case scenario, it will do so without damaging the piston at all, although a sudden engine failure in a car moving at any speed at all remains dangerous.
Hydrolock at high speeds
At high speed, however, hydrolock can be much more damaging. If a fast-moving piston hits a layer of liquid it can't pass through, it will come to a sudden halt. This usually results in the piston rod bending or even snapping. The cylinder head may also be damaged. Damage to the bearings or crankcase is also common. If the engine doesn't immediately stop -- for example, if only one cylinder is damaged -- a characteristic screeching sound from the piston may be heard.
In some cases, repairing the damage to a hydrolocked engine is relatively simple. If only minor damage occurred, the engine will not restart but can be repaired. The mechanic will expel the water from the system, usually by removing the spark plugs and running the starter to push the water out of the cylinder. In more severe cases, damaged piston rods, cylinder heads and even the crankcase may have to be replaced. It is often less expensive to replace the entire engine than to individually replace the components destroyed by a hydrolock.