The gasoline in your car began its life (for want of a better term) around 360 million years ago. That’s long before even the first dinosaurs wandered the Earth; despite the popular image of the typical automobile being “fueled by exploding T. Rexes”, it’s actually ancient algae and plankton. Given how long it’s been hanging around so far, then, it seems particularly unlucky for us that we got ahold of it so close to its expiration date.
How long does gasoline take to expire?
It’s true: whilst post-apocalyptic visions of the future like The Last of Us or Mad Max all seem to take place in worlds where years- or even decades-old gas is useable and valuable, the reality would probably involve less “jumping in an abandoned Chevy and making a speedy getaway” and more “trying in vain to start a gummed-up engine while breathing in one of the most disgusting odors you ever smelled.”
“Gas does have a shelf life,” confirmed Matt Crisara in a recent article for Popular Mechanics. “Left dormant in your vehicle’s tank, it can expire in as little as four weeks.”
With proper storage, that can be extended some: “you can expect anywhere from three to six months with fuel that’s been stored in jerry cans – in proper conditions,” Crisara explained, while “fuel stabilizers can boost the shelf life from anywhere between one to three years in optimal conditions.”
Why does gasoline go bad?
To understand the different ways that your fuel can go bad, we first need to take a look at what gasoline actually is – and that’s no simple task.
The stuff you pump into your car’s fuel tank is a very different substance from the crude oil that was pumped out of the Earth a few hundred million years after those algae blooms originally died. Sure, at its most basic description, it’s the same: it’s a mixture of hydrocarbons of differing weights, which can be combusted to provide energy.
However, in between that and the gas station, it goes through quite a few important changes. The heavier hydrocarbons are taken out, leaving the fuel as a mixture of paraffins (alkanes), olefins (alkenes), and cycloalkanes (naphthenes); impurities such as sulfur are removed in the refining process; and substances such as ethanol, anti-rust agents, and other things designed to improve vehicle performance are added.
It’s some of these additions which can cause one kind of problem with leftover gas. Ethanol, for starters: it’s put into the mix thanks to its high octane number, as well as its supposed ability to slightly reduce the carbon footprint of your fossil-fuel-guzzling automobile. However, it’s also hydrophilic – it likes to bond with water – and that can cause big problems for your car.
“If there’s ethanol in your gasoline, it could start sucking in water vapor from the air and putting it into your gasoline,” chemical engineer Richard Stanley told Live Science. “You don't want water in your engine, because it starts corroding the system.”
Then there are the olefins. As hydrocarbons with a double bond between two carbon atoms, these molecules are particularly susceptible to a process called oxidation – they start to react with the oxygen in the air, creating a nasty gum-like solid that can jam up your car’s engine.
“Once [the bad gasoline] gets into the pipeline, that gum may separate out […] and maybe [it will] not block the gas line fully, but maybe [it will] start to block it,” James Speight, an independent fuel and environmental consultant and author of over 100 books and papers on oil refinement and processing, told Live Science.
"You can almost say that gumming of the gas lines is like atherosclerosis," he added, likening the glooping-up of your vehicle to cholesterol plaque buildup in the body’s arteries.
That’s not the only way gasoline goes bad, though. Because the fuel is made up of only the lightest hydrocarbons out of crude oil – generally consisting entirely of those chains with 12 or fewer carbon atoms – leaving it too long can actually result in some of those molecules evaporating away. This can be particularly problematic if you’re trying to start your car in the summer using gas that’s been in the tank since winter, Speight advised: petroleum companies change the blends of hydrocarbons in their gasoline from season to season, so as to better cope with heat or cold temperatures – and winter fuel is way more likely to evaporate than its summer equivalent.
“If you leave gasoline by itself, over time [...] it just doesn't perform the way you think it's going to perform,” Stanley told Live Science.
Gasoline is "like wine,” he said. “Once you take it out of the bottle, it starts going bad.”
What to do if your gasoline expires
So you’ve decided to go for a drive, only to be faced with a tank full of muddy orange gunk that smells like – in Crisara’s words – “an old gym sock that’s been soaked in milk and left to rot for years.” What do you do?
According to UK automotive services company The RAC, it depends on how full your tank is. “If your tank is full of old fuel (especially old diesel) have it drained by a garage or a professional mobile service,” they advise.
If you’re running on a bit less than that, you might have a cheaper option. “If you suspect your petrol [aka gasoline] or diesel is stale the best advice is to try topping up with fresh fuel from a filling station,” they write.
Of course, the better tactic is to store your gas in such a way as to maximize its shelf life. “The main enemies for fuel storage are oxygen, water, and heat,” William Northrop, a professor of mechanical engineering at the University of Minnesota, told Popular Mechanics – and so, to increase your fuel’s longevity, you’re going to want to reduce the impact of those effects in particular.
To that end, it’s a good idea to store gas in a fairly full container, Northrop advised: “You’ll evaporate some of the volatile components,” he explained, “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.”
Even more important than a full container is its surrounding environment. Keep any gas you’re storing somewhere with a constant temperature and low humidity, Northrop advised – and remember: by its very nature, gasoline is really not something you want to treat carelessly.
“Remember, gasoline is very, very volatile,” Speight told Live Science. “It's not worth trying to store large amounts. It can just result in trouble.”
“Anything that makes the gasoline a little more volatile than it normally is affects the gasoline,” he added – joking that “on a hot day… [that can include] looking at the stuff the wrong way.”