Why Does Car Stop In Water? You need to know this


I’m a mechanic for over twenty years and I’ve worked on a ton of water-damaged cars. Most drivers don’t fully appreciate just how devastating driving a car through a simple water splash can be.

A car commonly stops in water because one or more of the engine cylinders fills with water. As water isn’t compressible, the affected cylinder’s piston stops dead, it’s a condition known as hydro locking. In severe cases, hydro locking may bend and break internal engine components. 

In this post, you’ll understand why water inside the engine is so devastating and you’ll learn some other common reasons a car stops in water. You’ll never look at a water splash in the same way again.

What is Hydro Locking?

Hydro locking is when an engine’s pistons can’t move inside the cylinder, because the cylinders are filled with a noncompressible fluid, usually water, but large quantities of coolant, oil or gasoline will also cause hydro locking.

To appreciate the dangers of hydro locking it is first useful to understand a little about the four stages, better known as strokes of an engine cycle. When understood, the dangers caused by water ingestion will become obvious.

All engines are configured around a central crankshaft. Controlled explosions inside cylinders drive pistons downwards which in turn powers the crankshaft. Each cylinder must pass through four separate strokes in preparation for its time to shine.

The four strokes of an engine:

Diagram of stroke 1 of a 4 stroke engine

1 Induction – The intake valve opens as the piston moves down the cylinder, fuel and air enter the cylinder, and as the piston reaches the bottom of the cylinder the intake valve closes.

Diagram of stroke 2 of a 4 stroke engine

2 Compression – Piston moves back up the cylinder compressing the fuel mix into the combustion chamber at the top of the cylinder.

Diagram of stroke 3 of a 4 stroke engine

3 Power aka combustion – As the piston passes TDC (Top Dead Center) the plug fires, igniting the mix which drives the piston down the cylinder powering the crankshaft as it moves.

Diagram of stroke 4 of a 4 stroke engine

4 Exhaust – As the piston moves back up the cylinder the exhaust valve opens to allow the spent gases to escape to the exhaust system, the valve closes before the piston reaches the top of the cylinder and the cycle starts once again.

Now let’s take a look at what happens when an engine ingests water. It should be noted that a simple water splash is rarely the cause of hydro locking, it is more usual when water is deep enough to cause a wall of water to cover the air intake assembly.

Diagram of engine ingesting water

Water enters the engine through the air intake duct, it passes through the air filter and throttle body, from there it’s sucked into the cylinder through the open valves of whichever cylinder is on the induction stroke. 

No harm has been caused yet, but it’s all about to hit the fan.

Diagram of engine bending con-rod

As the piston moves from the induction stroke to the compression stroke, the water is forced to the top of the cylinder. But as water isn’t compressible and has nowhere to go, the piston simply stops dead in the cylinder, like it hit a wall.

All that energy must go somewhere and usually transmits into the conrod (connects the piston to the crankshaft), causing it to bend or break. Pistons breaking and pistons breaking through the wall of the engine block are also very possible.

A bent conrod is replaceable, it’s an expensive repair, but it is preferable to a busted block. A busted block means a new engine and for many cars, it’s the end of the line.

Symptoms Of Hydro Locking

Chevrolet engine

Symptoms vary depending mostly upon engine speed, higher rpm just prior to water ingestion usually results in severe engine damage.

If on the other hand, the engine was at idle prior to water ingestion, the chances of no damage are pretty good.

Mild hydro locked symptoms include:

  • Engine stalls after hitting standing water
  • Engine won’t crank just clicks (like the battery is flat)
  • Engine won’t initially crank over, but after several attempts it does

Severe hydro locked symptoms include:

  • Engine stalls after hitting water
  • Metallic noises from engine 
  • Engine won’t crank just clicks (like the battery is flat)
  • Engine leaking coolant
  • Engine leaking oil
  • Hole in engine block
  • Engine runs but vibrates badly at idle
  • Engine runs but missing on a cylinder with check engine light on

How to diagnose Hydro Locking

Air filter

If your vehicle has ingested water and stalled while driving through standing water, avoid trying to restart the car. Hydro locking is the most likely reason the motor stalled. 

To diagnose hydro locking, pop the hood and check the airbox for water. A wet air filter and standing water inside the airbox indicate your engine has indeed ingested water. Attempting to start the motor now may cause more damage.

If on the other hand, there’s no water inside the airbox and the air filter is dry – you got lucky this time.

How To Fix A Hydro Locked Engine

Car engine coil over plug

Having confirmed your suspicion of hydro locking (water in airbox), and assuming you have checked and don’t have a big hole in the side of the motor, go ahead and remove the spark plugs, (diesel engine remove glow plugs).

Disable the fuel system by removing the fuel pump fuse, that’s important, otherwise, gas is fed to each cylinder and a stray spark could ignite causing a fire risk. Remove the coil plug connectors also, this reduces the risk of stray spark but also removes the risk of coil damage due to voltage overload.

Now crank over the engine having bystanders stay clear as water will shoot from the affected cylinders, (one or two cylinders usually).

Repeated cranking will clear the cylinders, now go ahead and clean, dry, and check the plug gap. Refit the plugs, coils, coil pigtail wiring, and fuel pump fuse.

Moment of truth, crank her up, if she runs smooth, congrats you are born lucky. If however the engine stumbles or vibrates badly especially at idle, you may have a bent conrod.

How To Check Conrods

Ok, but what are the conrods? The Conrods are metal components employed to attach the piston to the crankshaft. Hydro locking as you know commonly causes the conrods to bend, just slightly, but it’s enough to cause the piston to sit lower in the cylinder. 

It isn’t common for more than one conrod to be affected and of course, it will be the conrod belonging to the affected cylinder.

Set each piston in turn to top dead center (TDC). TDC is where the piston travels to the very top of its travel, just before returning down the cylinder. You can check this by placing a blunt object like a wooden dowel into the spark plug hole until it meets the piston. 

Turn the crankshaft using a wrench (easy once all spark plugs are removed) until the dowel reaches its maximum height, this marks the TDC.

Go ahead and mark the dowel where it meets the plug hole surface. Repeat this on all cylinders, a bent conrod will mean the dowel mark rests below the spark plug hole surface.

Usually, one and possibly two cylinders may be affected. This is a major repair as it will require a full engine strip down to fit new conrods. Mileage, age, and condition of the vehicle may mean this repair isn’t economically viable.

If your car is comprehensively insured, it may be a good time to use it.

Wet Ignition System

Plug Coil

While a wet ignition system isn’t ideal, you’ll find most modern ignition systems won’t be affected by dampness, that’s why these types ignition systems are so successful. Anybody old enough to remember distributor caps, plug wires and god forbid contact points will remember how common ignition system faults were.

Modern ignition systems are very close to waterproof, coils fitted to each cylinder are solid-state units with no exposed wiring to corrode. Pigtail wiring comes with a weather pack seal which keeps moisture out. So unless your car is pre-1990’s, I don’t expect an ignition system issue caused by dampness.

That said, modern but older cars suffer from electrical issues. These are often introduced by the previous testing where weather seals were breached or plug-in connectors broken.

Add to that the absence of older cars splash guards, they are often broken or simply not refitted for ease of future access.

The splash guard has an important function, you guessed it, it prevents water from forcefully hitting engine components. I have seen it many times where a wall of water simply dislodges an electrical connection which brings the car to a stop.

Check those connections and fuses.

Check out my favorite tools on the Mechanics tools page.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty-five years, and I've worked for GM, Volvo, Volkswagen, Landrover, and Jaguar dealerships. My passion is cars. I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of car ownership, including buying advice, maintenance, and troubleshooting.

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