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Can Gas Go Bad Sitting in a Car?

By Melissa Spicer | September 11, 2022

How Long Until Gas Goes Bad, Exactly?

Fuel can be stored for three to six months in a proper container in your garage, typically. Under the right conditions, though, and with the addition of fuel stabilizers, the storability for gas or diesel fuel increased by about 12 months.

Still, the gas inside your car’s fuel tank is not as forgiving and will most likely begin the degradation process in around a month if the engine is not fired up.

Should You Use Old Gas?

No, absolutely not.

During the deterioration and degradation process, fuel takes on a thicker, more solid composition, becoming “gummy.” This gunk entering your engine can cause major damage to your fuel system’s internal components, not to mention creating clogs in your vehicle’s fuel lines as it makes its way through.

Using old gasoline in your car results not just in reduced performance in the short term but may potentially necessitate repair work down the road.

Has Your Gas Gone Bad?

You can usually identify gas that has surpassed its prime by its look and its smell since the deterioration process causes it to undergo changes in its chemical properties.

Gas that has gone bad tends to be a darker hue than regular or pure gasoline, and if you take a whiff of it, you’ll notice a malodourous, sour scent.

You should also look for your check engine light to come on. All other problems aside—if your car has oil and is otherwise running okay, then its check engine light may be coming on because of improperly burning fuel. Have your mechanic take a look.

Operationally, bad fuel can cause your car to:

  • Lose power as you drive, particularly when you are accelerating

  • Sound rough when idling

  • Hesitate or even fail to start

If gas has been sitting in your car for a while and you notice these problems upon firing it up and taking a spin, then you may need to have it serviced. Typically, this involves removing the bad gas from the car’s fuel tank—hopefully, before the gummy residue it produces has time to damage fuel lines, fuel injectors, and your fuel pump. It can also create blockages and even cause expensive destruction to the internal engine components.

Keeping Gas From Going Bad

There are a few things you can do to help fresh gasoline last longer. One way to make it last for a longer period of time is to start your car and drive it from time to time, even if it is only around the block.

A few times each week is ideal for keeping the fuel inside your car’s tank from going bad. Also, this burns out the existing fuel, so you will have to add fresh gas to your car periodically.

In the same vein, when possible, keep your fuel tank full of new gas. This leaves less space for moisture in the tank, which contaminates the gas and eventually leads to oxidation.

What’s more, keeping your tank fuel can also limit the oxygen inside, which is what gas requires to go through the evaporation process.

Adding a fuel stabilizer can also help prevent the breakdown of fuel. Just add it in a full tank of fresh fuel if you plan to part your car and drive it less.

If you are going to store gas in a storage container (like a plastic container rated for fuel), only store a few gallons together at once. Store gasoline in a low-humidity, low-oxygen environment and at a cool temperature.

Factors Affecting the Shelf Life of Fuel

When it comes to gas going bad sitting inside your vehicle or even on a shelf in the garage, there is no one-size-fits-all rule that determines exactly how long it takes it to degrade to the point that it’s no longer useful or when it may even become harmful for your car.

Some factors that help influence how long gas is good for once stored in an inoperable vehicle or elsewhere, such as in your lawnmower or a sealed container, include:

  • How you store the gas

  • The ethanol levels in the fuel

  • The overall temperature and conditions of storage

  • The time of year the gas is lying dormant

Understanding Fuel

To better understand why gas or diesel fuel degrades at all, it’s helpful to understand fuel’s unique makeup. After all, gasoline derives from crude oil, and crude oil is made from decayed animals and plants from millions of years in the past, right?

While you’re spot on when it comes to the origin of crude, the truth is that when gas makes it through the refining process, it goes through massive changes. By the time you pump it into your car’s fuel tank at your local Shell or Exxon gas station, gas has become a complicated mixture of hydrogen and carbon items. This combination makes the combustion of hydrocarbons possible, which is how your engine runs in the first place.

And as crude oil makes its way from prehistoric sludge to high-performance fuel, it gets doused with all sorts of additives, including ethanol, which improves fuel performance, helping to power your car more efficiently. It’s barely recognizable when it arrives in the gas tank of your car.

Why Does Gas Go Bad?

As time passes, fuel begins to change its composition and degrade. This affects engine performance. Most of the changes that occur to fuel as time goes by are due to three environmental components, including oxidation, water, and evaporation.

Above, we briefly hinted at how the refining process makes the combustion of volatile compounds possible. Unfortunately, the same hydrogen and carbon atoms that power combustion and make your car go also play a role in what makes gas go bad. Oxidation.

As it turns out, as time passes, weeks, months, and longer, the oxygen molecules in fuel begin to affect the fuel’s and hydrocarbons, altering their very structure. This influx of oxygen into the hydrocarbon molecules increases the corrosion process, which in turn has a detrimental effect on the combustion process, making it less efficient. The result is what we call gas “going bad.”

Ironically, an additive that’s meant to increase fuel’s performance also factors into how long it takes gas to go bad. Ethanol.

All gasoline, at least in the U.S., has at least 10 percent ethanol in its composition, so the higher the octane the better. And while ethanol enhances fuel performance, it also attracts water molecules from the air, and as you know, water and gas are not an attractive mix. Water in fuel shortens the fuel’s potential storage life, and the more ethanol the gas has, the faster you can count on water vapors making gas “watery” and unusable.

Above we mentioned that the time of year you’re storing gas has a bearing on how quickly gas goes bad? That’s where evaporation comes into play.

The fuel industry has two blends of gas—a summer blend and a winter blend. Summer gas is made up of heavier hydrocarbons while winter fuel is made from lighter hydrocarbons. This is because lighter hydrocarbons evaporate more rapidly than their heavier counterparts. By switching up the composition for either summer or winter fuel, manufacturers help engines run more efficiently. The more fuel evaporates, the more it oxidizes and degrades.

The Bottom Line

Gas can and does eventually go bad when left in a car that’s never started or driven. How long it takes gas to go bad, however, is not entirely set in stone. Several factors can influence how long gas takes to go bad, including the ethanol levels in the particular gasoline and the time of year. As a general rule of thumb, gas sitting idle in your car’s fuel tank will begin to degrade within a few months, which can affect automotive performance and end up causing you to be out the cost of repairs for your car’s engine and fuel system.