A personal watercraft’s fuel tank is designed to have additional room into which fuel can expand. They leave space to prevent dangerous conditions like overflowing fuel, fires and explosions. For that reason, as an operator of a jet ski, you always need to know how much fuel your PWC can hold and how to properly refuel when necessary.

How to Use This Fuel Tank Safety Feature

Jet Ski vs WaveRunner vs Sea-Doo

Fuel, either diesel or gasoline, will expand in heat. Gasoline can expand by about 1% in volume for every 15 degrees Fahrenheit that the temperature goes up. A fuel tank in hot weather under pressure with a sealed cap can reach up to 140 F or more, depending on conditions, which can cause significant fuel expansion in an overfilled gas tank. This is why it’s important to never overfill the tank to the point of danger, as expansion will cause fuel to leak.

The average PWC fuel tank holds about 15 gallons of fuel. Some smaller models can only hold around 5 gallons while larger ones can hold over 20 gallons, however. For that reason, you definitely want to make sure you know the precise fuel capacity before ever trying to refuel any tank, but especially a PWC. Remember, unlike a regular boat with an outboard motor, there is very little separating you from the fuel tank in a PWC so you want to make sure you’re being safe.

If you’re using an automatic pump, the gas should stop flowing automatically when the tank is full and you know it’s time to stop. But if you’re using a gas can or are fueling up at an old dock station without an automatic shut off you’ll want to stop filling the tank when the fuel level reaches about two inches from the top. You can avoid fuel spills by not topping it off beyond this as that extra space is your safety feature and will ensure you’re less likely to suffer any serious repercussions.

Can Overfilling a Tank Cause Explosions?

This is an urban legend that surfaces every few years, especially during heat waves. Because hotter temperatures lead to expansion, people spread the false rumor that your tank can actually explode if it’s overfilled. However, for gasoline to ignite spontaneously, without a spark, would require temperatures over 500F and it’s literally impossible for that to happen just from the heat of the sun outside.

It is not usually easy to overfill a PWC fuel tank because of this safety feature but you may see some riders pulling some tricks to get a little bit more fuel in the tank. The most common method is to tip the personal watercraft to one side as you fuel it up, allowing the fuel to fill more of the tank in the process. This is actually even worse than just topping it off past the filled point.  If you were to look at a fuel tank from the side you’d see that the hole in which you add fuel is not directly on the top of the tank, rather it’s off set to the side. So, while you should only fill the tank to two inches below this spot, there’s actually about another two inches of space above this as well.

If you see a PWC rider tilting their vessel as they fuel up, this is a reason why. They’re trying to fill out that entire space.  Often people do it under the assumption they’re getting more bang for their buck because it allows them to stay out on the water longer but, as I said, this can be a real hazard. When the vessel is straightened again, if the cap isn’t on securely fuel will spill out. And if it is on securely, it just means the tank is far too full where it will start to heat up and expand.

The Danger of an Overfilled Fuel Tank

Kawasaki Jet Ski

Explosions are not a direct risk of an overfilled fuel tank in your PWC, but there is still danger. For one, you could potentially leak fuel and that’s a double problem. First, fuel getting into the water is bad for the environment and can kill marine life that is exposed to it. And second, the risk of fire is dramatically increased with leaking fuel. Even if the heat of the day can cause an explosion, exposed fuel is highly susceptible to reaching a spark or other heat source and igniting to cause a fire.

As the fuel in your tank expands, it can start flooding fuel system components. One way to tell you’ve overfilled the tank is that you’ll start smelling fuel even when there’s no visible sign of a leak anywhere. This can actually go on for a few days because the gas has expanded into places where it’s not normally found and the gas fumes are pretty long-lived. It may not be a fire risk, but you can’t really know for sure if it leaked somewhere inside the PWC either since the system is enclosed and hard to visually inspect.

Once you’ve fueled up, open the door to your engine compartment and see if you can smell gas. If you overfilled, you’ll likely notice the odor much more strongly than normal at this point. If you’re smelling anything, check for any leaks before starting the engine, just in case. The risk of a fire is very real and something you obviously want to avoid. This is one reason why you need to make sure your PWC has a fire extinguisher on board that’s in good working condition.

The Bottom Line

Jet Ski, Sea Doo and other manufacturers include extra space designed into PWC fuel tanks. This allows gas to expand and vapor to fill the space as it heats up. If a tank is overfilled, then there’s a risk of gas being forced out and potentially a risk of fire. Though a fuel tank will not spontaneously explode if overfilled, it can still pose a risk of fire thanks to fuel spills or leaks. Carry extra gas for jet skis in jerry cans if need be