Why Is My Gas Can Expanding or Getting Cold?

Why a gas can swells

Gasoline gives off vapor all the time, and that vapor mixes with the air in the empty part of the can. When the can warms up, that mix of air and vapor expands. Gasoline itself also evaporates faster when it is warm, so more vapor adds to the pressure.

A sealed plastic can has nowhere to send that extra pressure, so the walls bulge outward. Plastic is flexible, so you see it more clearly than you would on a rigid steel can. The usual triggers are sun on a truck bed, a hot shed, or a garage on a warm afternoon.

Swelling is not a sign that the fuel is bad. It is a sign the can is sealed well and the temperature changed.

Why it can shrink or collapse later

The same thing runs in reverse. When evening comes and the can cools, the vapor and air contract and the pressure drops. If the can was sealed tight, the walls can pull inward and look dented or sucked in.

Repeated swelling and shrinking works the plastic. Over many cycles it can leave a can warped or stressed around the seams and spout. If a can stays out of shape after it returns to room temperature, or you see cracks or seeping fuel, retire it.

Why a gas can gets cold

Evaporation takes heat from whatever is left behind. When gasoline turns to vapor, it pulls heat from the liquid and the can wall, so the container can feel cool to the touch. You notice it most when you open a warm, pressurized can and vapor rushes out, or while you pour and fuel evaporates quickly.

There is also a simpler reason. A can stored in a cool garage, basement or shed will feel cold if the room is cold, and fuel holds that temperature for hours.

If you mean a propane or other pressurized gas canister, the idea is the same. As liquid fuel boils into gas, it chills the metal, and you may even see frost on the outside during heavy use. That is normal for those canisters, but it is a reason not to hold a bare cold canister for long.

What to do about a swollen can

Treat a bulging can as if it holds pressurized fuel vapor. Do not rush to open it. Move it out of the sun, set it outdoors on the ground away from any ignition source, and let it cool.

Follow these steps once it has cooled:

Always wear safety glasses, because pressurized vapor and fuel can spray when a cap is released. Do not smoke, and keep pilot lights, grinders, sparks and running engines well away. Open caps slowly and point them away from your face. Never puncture a can or squeeze it to relieve pressure.

• Move the can to a shaded, open outdoor spot, away from the house, vehicles and anything that could spark.
• Wait until it is cool to the touch.
• Loosen the vent cap or spout slowly, a little at a time, so vapor escapes gradually rather than in a burst. If your can has no separate vent, loosen the main cap just enough to hear the pressure release.
• Let the can settle, then reseal it once it has equalized.
• Check the can for cracks, leaks or permanent bulges. Replace it if it is damaged.

How to prevent it

Most swelling problems come down to temperature and fill level. Store cans in a shaded, ventilated place that stays fairly steady in temperature, such as a detached shed or a garage away from heaters and water heaters. Never store fuel in living space or in a car trunk on a hot day.

Do not fill a can to the brim. Fuel itself expands when warm, so leave the headspace that the can's fill line or manufacturer instructions call for. Keeping a can mostly full also leaves less air to expand, but never past the marked fill level.

Venting before big temperature changes helps too. Releasing built-up pressure carefully, outdoors, can keep walls from stressing. Using only approved fuel containers matters, because makeshift jugs are not built for pressure or vapor.

Does can design matter?

Yes. Metal safety cans resist bulging because the walls are rigid, and many include a spring-loaded cap that opens to vent internal pressure when needed. Plastic cans give more visible movement, but they are lighter and resist rust and dents. Our best gas can comparison lists type, material, weight and venting features side by side, so you can match a can to how and where you store fuel.

Here is how a few cans on that page compare, using their published specifications:

If swelling bothers you in a hot storage spot, a metal can with self-venting behavior is worth a look in our gas can picks. If you lift and carry cans often, remember that gasoline weighs about 6 lb per gallon, so a full can is heavy on top of the empty weight listed below.

Frequently asked questions

Is a bulging gas can about to explode?

Unlikely under normal conditions, but treat it with care. Cool it in the shade outdoors and release pressure slowly, away from sparks and flames.

Can I leave a swollen can in my garage?

It is better to cool it first, vent it, and store it away from heat sources and living areas. Check it for cracks before reuse.

Why does a gas can feel cold right after I pour?

Fuel evaporating from the can and spout takes heat with it, so the surface and remaining fuel feel cooler than the air around them.

Does a dented or collapsed can still work?

It may, but repeated shape changes weaken plastic. If the can leaks, cracks or does not hold its shape, replace it.