Car Stalled in Water: Don't Restart It

What hydrolock is

If the engine draws in water instead of air, that's called hydrolock. Water can't be compressed, so the engine stops abruptly. This can bend connecting rods, break valves, and in the worst case crack the engine block. Typical signs are an engine that won't restart after driving through water, or one that cuts out mid-drive.

Do this immediately

1. Switch off the engine if it's still running, and don't restart it. A start attempt can significantly increase the damage.
2. Switch on hazard lights and get out if the spot is safe. If the water is rising, leave the vehicle before the doors can no longer be opened.
3. Don't wade through moving water. Even shallow water can sweep you off your feet.
4. Call for help: 112 if anyone is in danger, otherwise a towing service.
5. Take photos: the water level, the vehicle, the surroundings. You'll need them for your insurer.

Why the car shouldn't drive itself

A vehicle that has drawn in water can look fine on the outside. The damage is inside the engine. Don't tow it with the driven wheels turning, since that spins the engine over. Transport on a flatbed is the safe solution, especially for electric and hybrid vehicles. For those, water can also affect high-voltage components and connectors, see Towing an electric car.

Informing your insurer

Report the damage to your insurer before the vehicle is repaired. Whether and how much is paid depends on your policy and the circumstances, for example whether it was a flood or a deliberate drive through flooded roads. The garage should document the damage and establish the cause.

What to tell the towing service

• Where the vehicle is and how deep the water is
• Whether the water is still rising or the road is closed
• Vehicle and drivetrain
• Whether the engine was started again after contact with water
• Whether the doors and locks still work

If your car is in water in an underground garage or underpass, our guide Underground garage and car park can help.

Preventing it

Don't drive through flooded roads. If it's unavoidable, drive slowly and steadily so no bow wave forms, and keep the engine revs constant.

Wading depth and the air intake

Every vehicle has a maximum wading depth, stated by the manufacturer in the handbook. It mainly depends on where the engine draws in air. If the intake sits low, even a puddle can be enough for water to reach the engine. Water can't be compressed, so the engine can be seriously damaged when it draws it in, with connecting rods breaking in the worst case. If the engine stalls while driving through water, that's often the exact moment the damage happens or is about to happen.

What water does to the electrics and electronics

Modern vehicles have control units, wiring harnesses and sensors located throughout the car, some in footwells or under seats. Water can corrode connectors and damage control units, even if the engine itself was untouched. On hybrid and electric vehicles, the high-voltage system is also affected, and special caution applies, see Towing an electric car. A vehicle that's been standing in water shouldn't be put back into use until a garage has checked it.

The oil reveals it

A sign of water inside the engine is milky-grey oil on the dipstick or in the filler neck. Rough running or water in the air-filter housing are also signs. Tell the garage about these findings, since they determine the scope of the repair.

Don't push-start or roll-start it

The vehicle shouldn't be pushed or roll-started to start the engine. The momentum can damage the engine further if water is sitting in the cylinders. Automatic transmissions shouldn't simply be towed either, see Towing an automatic car. The safe route is transport on a flatbed.

After the water: what to document

• Photos of the vehicle and surroundings showing the water level, as far as it's safe to take them.
• Time, place and weather conditions.
• Whether the engine was running when the water got in, and when it stopped.
• Invoices for transport and inspection, since they can matter for your insurer.