Yes, Gas Goes Bad: Why Gasoline Isn’t Shelf Stable

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This is why your fuel goes bad, according to the experts. (Plus: how you can maximize its shelf life.)

Hollywood movies and TV shows often portray apocalyptic landscapes in which heroes hop into automobiles that not only have a full tank of gas, but also fire up instantly after laying around dormant for years. More recently, you may have seen Joel from HBO's The Last of Us drive off in a beater that's been sitting for 20 years. In reality, he would have to think of an alternate mode of transportation because the old gasoline in all of these cars (and really any other gasoline on the planet) would be rendered useless.

Yes, we’re aware that Joel siphons supposedly fresh gas from other vehicles parked at a nearby gas station, but this still wouldn't really work. Gasoline has a limited shelf life just like fruit or other perishable foods you’d find at your local supermarket. It will only last for about six months—if kept in ideal conditions—or possibly up to a year if a fuel stabilizer is used, but nowhere close to the 20 years that The Last of Us would have demanded.

✅ Keep reading to find out why gas expires, and how you can maximize the shelf life of any gas that you’re storing for extended periods of time.

Gasoline as we know it is an amalgamation of roughly 150 different types of hydrocarbons. While "hydrocarbons" might sound complex, they’re merely compounds that contain hydrogen and carbon.

That being said, modern gasoline is more complicated than it seems. It has to act not only as a fuel, but also as a lubricant, along with anti-rust and anti-icing properties to help keep your engine running properly. Most of the fuel you buy will also contain ethanol, which can be problematic if left alone for extended periods of time.

It's true: gas does have a shelf life. Left dormant in your vehicle's tank, it can expire in as little as four weeks. Meanwhile, you can expect anywhere from three to six months with fuel that's been stored in jerry cans—in proper conditions. Motorcyclists and classic car enthusiasts will know that fuel stabilizers can boost the shelf life from anywhere between one to three years in optimal conditions.

Unfortunately, there are multiple ways that gas can expire: through oxidation, evaporation, and taking on water.

Gasoline is prone to oxidation when sitting stagnant for extended periods of time. Keen mechanics know that old gasoline develops a gum-like film; it's very similar to how pennies become discolored when they’ve been left outside for too long. However, unlike pennies or other metals that don't get along with oxygen, gasoline contains a particular set of hydrocarbons called "olefins"—hydrocarbon components with a double bond between two carbon atoms—that are particularly susceptible. When these olefins begin to oxidize, they solidify into a gum-like polymer that is definitely not what you want to run an engine on.

William Northrop, a professor of mechanical engineering at the University of Minnesota (whose research focuses on fuels), says that in addition to these polymers burning poorly, they also gum up important components of your fuel system and engine—ie: fuel filter and fuel injectors.

The same gum-like films (oxygenated hydrocarbons) in your old gasoline are also a big contributor to its putrid smell.

We’ve been over the fact that gasoline is an amalgamation of hydrocarbons. However, the dinosaur juice that your car runs on also includes a number of compounds that are much more volatile than the others. "The lightest hydrocarbons, referred to as ‘light ends,’ volatize and get evaporated," Northrop tells Popular Mechanics. In fact, these light ends are largely responsible for starting your engine, which is one of many reasons why your car won't start—or might struggle to get going—with old gas in the tank.

If shows like The Last of Us or The Walking Dead were true to life, none of the cars would start, and the characters would all be doomed. Not very entertaining is it?

While oxidation and evaporation are the two biggest culprits for old gas going bad, ethanol brings in even more susceptibility—and yes most of the gasoline you buy nowadays will contain trace amounts of ethanol. So what's the big deal with this so-called corn juice?

The big ploy with ethanol is that it's a renewable type of fuel (or fuel additive in this case) that also oxygenates your gasoline, which reduces air pollution. However, ethanol brings a whole host of issues when it comes to storing gasoline; the biggest miss is that it's hydrophilic, meaning it likes water and has a tendency to attract it, and gasoline is hydrophobic. Therefore, if you store gasoline in a humid area (or really any area for that matter), it will absorb water, which can prove problematic in large amounts.

That's not to say you can't run an engine using watery gasoline: in fact, BMW's F82 M4 GTS actually used water injection to cool the charge air going into the cylinder head—which actually produced more power. Too much water, though, and your fuel lines will get rusty, and you’ll actually inhibit combustion.

Thankfully it's actually quite easy to know when and if your gas has expired. First off, it will begin to smell bad—honestly, old gas is one of the worst smells I’ve ever experienced: think of an old gym sock that's been soaked in milk and left to rot for years. Smelling salts be damned.

Less obviously, fuel will begin to go from its normal green/yellow color to more of a muddy orange. The gum build-up that we talked about earlier is also very obvious to see if your gas is stored outside of your vehicle's tank—especially in a jerry can or other translucent container.

"The main enemies for fuel storage are oxygen, water, and heat," says Northrop.

He mentions that it's best to store gasoline in a fairly full container; the idea is that the headspace at the top of the vessel will actually help curtail the evaporation process. "You’ll evaporate some of the volatile components … but once the concentration of those volatile components gets high enough in the vapor, they no longer want to evaporate because they establish an equilibrium between their vapor phase and their concentration of liquid," he says. Along with a mostly-full container, it's imperative to keep it in a controlled environment at a constant room temperature with low humidity.

One great way to extend the shelf life of your gasoline is to use a fuel stabilizer. We could write an entire article about how they work, but their main target is to combat the oxidation phase of gasoline going bad. "Some of the work we do with fuels that are very prone to react with oxygen during storage involves adding antioxidant chemical components that prevent the formation of peroxides … which are very reactive," David Rothamer, a professor of mechanical engineering at the University of Wisconsin-Madison, tells Popular Mechanics. Northrop also remarked that fuel stabilizers establish a vapor barrier.

Whether you’re storing a motorcycle over the winter, a weekend car in your garage, or just a jerry can of fuel for your lawn mower, proper storage is the key to getting your gas to last. Better to be proactive than to be caught with your pants down the next time you’re going for a ride, a drive, or escaping an oncoming hoard of zombies.

Matt Crisara is a native Austinite who has an unbridled passion for cars and motorsports, both foreign and domestic, and as the Autos Editor for Popular Mechanics, he writes the majority of automotive coverage across digital and print. He was previously a contributing writer for Motor1 following internships at Circuit Of The Americas F1 Track and Speed City, an Austin radio broadcaster focused on the world of motor racing. He earned a bachelor's degree from the University of Arizona School of Journalism, where he raced mountain bikes with the University Club Team. When he isn't working, he enjoys sim-racing, FPV drones, and the great outdoors.

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