Hydrolock is a situation where a fluid, usually water, fills a combustion chamber to a point where combustion stops or compression cannot occur. Severe damage to the connecting rods and engine seizure can occur.
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If a vehicle becomes submerged in water, drives through a flooded road, or through a deep puddle and stalls immediately after and will not start again, than hydrolock may be the cause. When an engine is running there is a constant vacuum through the air filter and intake system. When operating normally it is breathing in air and compressing it into the engine to aid in combustion to create power for the engine. Fluids however are not compressible. Meaning that if an engine is running and water is somehow sucked in through the intake and into the combustion chamber the water will not compress. An engine running at 3000 RPM can to come to a complete stop almost instantaneously if only one combustion chamber is filled with water. This will bend or break connecting rods or crank shafts, and can even break cylinder heads.
Leaking Head Gasket
A severely leaking head gasket can leak enough coolant into the combustion chamber and cause hydrolock. Other symptoms may occur beforehand though, tell tail signs of a leaking head gasket are, lack of cabin heat, coolant loss but no visible leaks in the cooling system, misfire on 1 or 2 cylinders from coolant in the combustion chamber drowning the spark plugs, and white smoke/steam from the exhaust pipe on start up or at idle as the issue grows. All of these symptoms will get worse over time, and sometimes although rarely it can happen very suddenly with older cars around 15+ years old. Leaking headgaskets will not stop leaking, as the saying goes, "rust doesn't sleep", it's typically best to tackle the issue immediately.
An engine that is experiencing hydrolock from a flood of external water from a puddle or from being submersed and getting it sucked in through the air intake will typically stall immediately afterwards and not restart.
An engine suffering from a leaking head gasket on its way to creating hydrolock will misfire, have white smoke pooring from the tail pipe from the burning coolant, and in extreme cases even have raw coolant pour out of the tailpipe. If the head gasket blows out suddenely instead of the typical slow deterioration over time, than it can cause a sudden ingestion of coolant which can cause the engine to stall out and or stumble, followed by the above symptoms associated with a bad head gasket.