What is a Hydrolocked Engine?

You may have seen all-terrain vehicles crossing streams and rivers without so much of a fuss. The waterline is already...

You may have seen all-terrain vehicles crossing streams and rivers without so much of a fuss. The waterline is already at the vehicle’s windshield, yet they keep on going. Once they reach the other side, they continue conquering the roughest trails. So, when the city streets get flooded, you have this idea that your car can do the same feat. After all, what damage could several inches of water do to your vehicle, right? But then, as you are traversing the flooded streets, your engine begins to sputter. Not long after, the sputtering sound gives way to loud knocking noises from the engine bay. Within seconds, you’re stuck in the middle of the flooded street and unable to restart your engine. Bad news: You have just hydrolocked your engine.

Hydrolocked Engine

What is a Hydrolocked Engine?

Short for hydrostatic lock, hydrolock is an abnormal condition in any device that compresses gas. Instead of compressing gas, it tries to compress liquids. Owing to the nature of liquids like water, no device can ever compress such a substance.

When applied to a modern internal combustion engine, it talks about the inability of the pistons to compress the air-fuel mixture. Instead of the cylinders containing air-fuel mixture, what is present is water. It is very difficult for the pistons to compress water within the cylinder since there is nowhere else to go. Hence, the pistons are unable to “move”, locking them into their position.

In simplest terms, hydrolock prevents the pistons from ever reaching their optimum stroke distance. The amount of water present in the cylinder should be more than the actual capacity or volume of the cylinder. This is at the level of the piston in its maximum upstroke travel.

What Happens in a Hydrolocked Engine?

To understand what happens in hydrolock, it is important to review how combustion engines work.

Combustion engines produce power by compressing a mixture of fuel and air in the cylinders. Pistons attached to the crankshaft move up and down in the cylinder. The upward stroke of the pistons compresses the fuel-air mixture. With the addition of a spark towards the end of the compression stroke, combustion takes place. This is in gasoline engines. Diesel engines work almost the same way, except that the piston compress air to heat it up. With the addition of fuel, it ignites in a spontaneous manner without the need for a spark.

The modern combustion engine can compress gaseous mixtures, but not liquids. Water is a non-compressible matter. When there is sufficient water in one or all of the cylinders, the pistons will no longer be able to move. This often occurs on the compression stroke.

What Causes It?

Water that enters the air intake is the most common cause of a hydrolocked engine. It can happen to vehicles that run on high flood waters or get immersed in water. Hence, driving the car in flood that is higher than the maximum tolerable flood limits by the car manufacturer can increase the risk of hydrolock. Water can enter the cold air intake system and the engine’s cylinders.

There are also instances when the flood water is not that high. However, following another vehicle that is making a large wake behind it can cause water to enter the air intake system. This is the reason why one should maintain a safe distance when following another vehicle on flooded streets.

Water is not the only reason an engine can experience hydrolock. A bad or blown head gasket can also lead to hydrolock. The fundamental job of the head gasket is to keep the cylinders safe from other engine fluids like oil and coolant. Hence, engine coolant can empty into one of the cylinders. If the amount of coolant entering the cylinder is large enough, then it can also cause hydrolock.

Another less frequent cause of hydrolock is a malfunctioning carburetor or fuel injectors. These can “inject” large amounts of fuel into the cylinders instead of the usual mist.

What Types of Damage Can Hydrolock Cause?

The extent of the damage to the engine depends on the engine speed when the hydrolock occurred.

If the engine was running at idle or at low RPM, then it is possible that there is no damage to the engine at all. Your only issue will be a stalled engine. In such a case, you will need a towing service to help you bring your vehicle to the nearest shop. Once there, the mechanics can start repairing your hydrolocked engine by removing the water from the cylinders. Time is of the utmost importance. If water stays so long in the cylinder, it can lead to corrosion and pitting. If this occurs, then you end up with expensive garbage.

If the engine was running at high RPM or at high speed when the hydrolock occurred, then you may have a catastrophic failure. Think about it. The pistons connect to the crankshaft via a connecting rod. Putting an immediate full stop to the rotational motion of the crankshaft can lead to cracks. It can also bend the connecting rod. In certain cases, the cylinder walls can crack or fracture. The cylinder head will blow its oil seals.

If the engine is running at full speed or at its maximum RPM, hydrolock can break the connecting rod. If this happens, it is possible that it will pierce right through the engine block.

Hydrolock can also occur when the engine is not running. For example, you parked it outside and there’s a flash flood. Floodwater can still enter the cylinders. If allowed to sit for a long time, then you end up with a severely-corroded engine core.

engine closeup

How Can Hydrolock Be Fixed?

Hydrolocking engines is easy. You only need to place water or any other fluid in at least one of the cylinders. Fixing hydrolock may not be as easy, though. Everything depends on whether there is any damage to the engine and/or its components.

If No Engine Damage: Engine at Idle or Low RPM

If the hydrolock occurred while the car is idle or running at low speeds, then it is possible to apply a very quick fix. This entails the removal of the water that is present in the cylinders. However, it is best that this is done in an auto shop so you will have the right tools in case something goes wrong.

Removing the water from the cylinders requires the removal of the spark plug. In some cases, mechanics also remove the fuel injectors. This will help provide an outlet for the water to escape from the engine cylinders. Crank the engine. You will notice water spewing out of the cylinders. At some point, it is possible to see gas coming from the exhaust. This is normal because of the extraordinary mixture in the cylinders. It should disappear fast enough.

With the removal of water from the cylinders, you can proceed to cleaning the cylinder walls. This is very important since moisture left on the walls can lead to corrosion. It is also necessary to complete an oil change and replace the spark plugs with new ones. This should help you get your car back on the road again.

With Engine Damage: Engine at High RPM

If hydrolock occurred while the engine is at full throttle, then there is no other solution but to bring the car to the shop. Mechanics will pull down the combustion engine and strip it down. It is critical to inspect every component and part of the engine for signs of damage. In most cases, there will be bent connecting rods, cracked pistons, or scored bearings.

It is also ideal to pressure test the cylinder head. Moreover, it’s necessary to inspect the engine block for any signs of cracks or deformities. The crankshaft also requires accurate measurement, making sure that this critical part of the engine is not bent.

Depending on the extent of the damage, you are looking at an expensive engine rebuild. In most cases, it makes better sense to purchase a second-hand engine and install it on the vehicle. This is more cost-effective than spending thousands of dollars rebuilding or fixing a severely-damaged engine.

How Much Should I Prepare to Have it Fixed?

The cost of fixing a hydrolocked engine often runs in the thousands of dollars. Without engine damage, you may only spend a few hundred dollars for the change oil and spark plug replacement. This also includes labor.

However, if you want to be sure about the integrity of your engine, you may still require the mechanic to pull down the engine and give it a very thorough inspection. This can set you back for a few hundred dollars more.

With extensive engine damage, you are looking to spend anywhere between $3,000 and $10,000. This depends on whether you’re going for an engine rebuild or an engine replacement.

A hydrolocked engine is never a good thing. As such, if you can avoid turning your car into an amphibious vehicle, then you will not have to worry about such problems.


  1. Hydrolocked Engine – What Is It – What Damage Can It Do – Danny’s Engine Portal
  2. What Gets Damaged in an Engine Hydrolock? – It Still Runs