Can A Car Engine Work Underwater? Here's What Happens When One Is Fully Submerged

If you've ever driven a car into water that was deeper than you expected, you might have suffered the consequences of a submerged engine. While getting a car engine wet is no big deal, getting water inside is a big deal. If the water level is above your car's air intake, the engine will stall, with several factors contributing to its demise. Firstly, an engine that cannot draw air cannot run. Internal combustion engines work by combusting fuel — burning or combining it with oxygen — so when the air intake is blocked, it suffocates and stops.

A submerged engine will also stop when the combustion chambers fill with water. This leads to a hydrolocked engine, and the process can be slow if the car is parked and flooded, or sudden and dramatic if water is sucked into a running engine. Unlike a fuel-air mixture, water is incompressible, and can bend and destroy things as it brings those moving parts to a sudden halt. 

If water gets into an oil sump, it can turn lubricants to sludge. Just 1% of water inside the engine oil can reduce the life of journal bearings by 90%. Water can also damage wiring, short out the ignition, and brick the on-board computer. Furthermore, if the exhaust pipe is underwater, the gas may not be able to escape. However, if air is coming in via a snorkel, and the exhaust pressure is greater than the water pressure, an engine can happily burble away underwater — provided everything else is waterproofed.

An internal combustion engine can work underwater, but it must be designed or modified to do so. While amphibious military vehicles have a long and illustrious history, passenger car manufacturers are less concerned with beach landings and river crossings. Some SUVs and off-roaders can be driven with water over the hood, or even submerged if the snorkel is high enough — but their higher price tags reflect the extras needed to survive fording rivers and streams.

A submersible engine could be fitted with oxygen tanks, which is how rocket engines burn fuel in space, but a simpler option is to fit a snorkel. Electronics and connectors must be waterproofed, with breathers for the fuel tank, crankcase, and transmission sealed or vented at the same level as the snorkel. Any cavity that can take in water should also have a drain.

There is a persistent myth that a diesel car can run underwater, while a gasoline car cannot. But as both are internal combustion engines, both have the same limitation, and that's the air intake. Both types of vehicle will require a snorkel. A diesel engine uses compression to ignite fuel, while a gas engine relies on spark plugs and wiring, and it is likely that this extra degree of waterproofing fuels the mythology. An electric vehicle is another kettle of fish: While its motor does not require an air intake snorkel, submerging lithium batteries in water poses a significant risk.

If your vehicle is not up to the task, submerging it in water can cause irrevocable damage. Even driving a properly equipped vehicle into a flooded road can cause loss of control, significant damage, and put lives at risk. Unseen parts of the road may have washed away, while the force of a fast-running river can act like bulldozer, forcing the vehicle off a submerged road or causeway. 

Most rental car companies stipulate their vehicles must not be driven into water — and for good reason. Water has a habit of getting into places it is not meant to go — not only the engine, but transmissions, differentials, and fuel tanks. Water can damage switches and wiring, while mud and debris can clog mechanisms including seat belts and air bags. Insurance companies will routinely declare flooded vehicles unrepairable write-offs, unable to be registered in most states. 

If you have driven a car into water, and are handy at wrenching, you can try to expel water from a gasoline engine by removing the spark plugs and cranking the engine over to force it out through the plug holes. Drain the oil, and if a lot of water is present, take further action by removing the valve cover and oil pan for inspection. You may need to take off the intake and exhaust manifolds, and remove the cylinder head, and, depending on the extent of the damage, even replace the engine altogether.