What Gets Damaged in an Engine Hydrolock?

Hydrolock Explained

If an engine’s piston cannot complete its full travel cycle -- up to its minimum at the top of its stroke, then down to its maximum at the bottom of its stroke -- the engine cannot turn over. If the piston or pistons that cannot complete their travel cycle are prevented from doing so by a liquid above them, they slam to a halt on the upstroke, hence the “lock” in hydrolock. This condition occurs when fluid enters the top of the cylinder, rather than gas; gas is compressible and the piston can keep moving upward as it compresses, while fluid is not compressible and prevents the piston from reaching the top of its stroke. Water was called “hydro” in ancient Greek, hence the “hydro” in hydrolock.

Causes of Hydrolock

Outside water can enter the engine through the air intake if the vehicle is driven through a flood; vehicles designed with low-mounted air intakes can draw in water from a bow-wave caused by driving through relatively shallow standing water. Engine coolant can enter the cylinders when a head gasket blows. A failure in the carburetor or injector mechanisms can introduce liquid gasoline where only a vapor-air mixture should be.

What Gets Damaged

Although an engine that hydrolocks at idle may simply stop, catastrophic engine failure is likely if an engine hydrolocks while running at speed. The most common result of hydrolock at speed is that the piston rods are deformed; they bend and fold between the piston at their top, which cannot travel upward any farther, and the crankshaft at their base, which continues to travel upward. Absorbing the force of the sudden stop may crack the block, crack the crankcase, destroy the head and shatter the bearings.

Manifestations and Rectification

Typically, an engine will seize solid if it hydrolocks at speed. If only one piston hydrolocks and the engine continues to move, there will be a loud screeching noise. Given that most of the major internal components have been destroyed, replacing the engine is typically more cost-effective than rebuilding.

If an engine hydrolocks at idle, it may simply stop and refuse to turn on the starter motor. There may well be no internal component damage. Rectification is by removing the spark plugs or injectors then turning the engine on the starter motor; this will expel the liquid from the cylinder or cylinders. Once reassembled, the engine should start as normal. The hydrolock, however, was a symptom, not a cause. If the liquid was introduced to the cylinder through a failed component, typically the head gasket, this must be diagnosed and rectified. Further, water is corrosive to the internals of an engine. If water has been inside a standing engine for any length of time, it could have caused rust bands to form inside the cylinder. These would have to be addressed, and the pistons perhaps replaced.