The short answer is yes: your gasoline can definitely expire and go bad. It can also become contaminated if stored improperly.

As a rule of thumb, stored gas can last between three and six months (or up to a year by adding a chemical such as STA-BIL Fuel Stabilizer). Like milk, there’s even a quick sniff test you can do: sour smells mean that the gas is old.

But the long answer to the question “does gas go bad?” is that it depends on many different things, such as ethanol levels and even the time of year.

The short answer is yes: your gasoline can definitely expire and go bad. It can also become contaminated if stored improperly.

As a gas-powered machine owner, engine health is the main concern around the “freshness” of your gas. Good gas means less wear and tear, which translates to a longer life and a fatter wallet at the end of the day.

To understand exactly how gas can go bad and how to prevent or deal with it, read on.

What is gas made of, and how does it go bad?

Gas is a complex cocktail of molecules that like to combust. These are hydrocarbons, and they come in many flavors. Those trained in chemistry will know some of them as alkanes, alkenes and cycloalkanes. Modern refineries add a whole bunch of other stuff to gas as well, but the stars of the show are the hydrocarbons. Their combustion helps make the engine run. Spark plugs add heat to compressed fuel and air, which blows up, causing a lot more heat. This abundance of heat expands the air, which drives the piston up, which turns a crank, and so on and so forth.

Gas degradation: oxidization

Oxygen is a main ingredient in this combustion, but it is also the cause of oxidation. If combustion is fast and exciting, oxidation by comparison is more slow and creeping. Over time, wandering oxygen molecules infect healthy hydrocarbons and change their structure. These oxidized molecules are far less efficient at combustion.

Gas degradation: water

Long story short, you do not want extra water in your gasoline cocktail. Go beyond the natural amount, and you’ll have a bad time. There’s one major culprit that likes to invite water to the party: ethanol.

Because of the way it is (chemical term: hydrophilic), ethanol attracts water molecules in the air around it. Most gas in the US has about 10% ethanol, classified as E10 for short. Because ethanol also oxidizes faster than other hydrocarbons, it reduces the shelf life of gas considerably. A good rule of thumb is that E10 gas lasts about three months before becoming questionable. Higher blends like E20 expire quicker.

Gas degradation: evaporation

Some hydrocarbons are heavier than others. The lighter ones tend to evaporate over time, which changes the cocktail mix. While this isn’t bad in and of itself, your motor may not be rated for a gas with heavier hydrocarbons, resulting in inefficient combustion.

In fact, gasoline sold at pumps will change their blend twice a year to adjust for hot summer days and cold winter ones. Summertime gas has heavy hydrocarbons to reduce evaporation. Winter has lighter ones for better evaporation (to make the engine run better). If your gas has evaporated its light hydrocarbons away, then you’ll find you run into more problems in the winter.

Identifying expired gas

There are some quick ways to test your gasoline’s health. “Bad” gas tends to have a darker colour and give off a sour scent. We recommend getting a sample of fresh-pumped gasoline to test against.

Unfortunately, aside from doing specialized chemical tests the only other way to tell is if your engine complains. We do not recommend this method of testing.

Preventing gasoline degradation

A ounce of prevention is worth a pound of cure, right? There are few things to keep in mind when seeking to preserve the lifespan of your gas, apart from using it on a regular basis.

  1. Proper storage. Oxygen and water are the chemicals that wreak the most havoc on your gas. Airtight, clean, gasoline-approved storage containers are your best defense. Keep them at room temperature.
  2. Avoid high ethanol blends if you’re not using the motor frequently, unless the specifications call for a high ethanol blend.
  3. Understand seasonality. Simply put, you need fresher gas in the winter. Technically, you need gas with lighter hydrocarbons, and old gas almost always has fewer of those. You should structure your buying and storing habits around this fact.
  4. Use a fuel stabilizer. Sometimes there’s no choice but to store gas or a gas filled machine for a long time. In this case, do not drain the tank! Instead, use a chemical like STA-BIL fuel stabilizer as directed. It will add new elements to our cocktail, helping to significantly slow down the oxidation and water contamination processes. (Watch our quick video to see why you should not drain your tank.)

Using expired gas

If you’ve come to the conclusion that you indeed are the sorry owner of some expired gas, all is not lost! If you mix the old stuff with new gas, it should work fine and without a huge risk to your engine. For smaller engines, a 50/50 mix works. For something like a car, 25% old and 75% new should do the trick.


Gas does go bad, thanks to being full of complex organic material (hydrocarbons). It’ll turn darker and start to smell sour once it’s well past its prime.

Keeping gas away from extra oxygen and water is key to prolonging its life. Newer ethanol blended gas will likely start to turn in three months or less, even in the best of conditions. Make sure to have some fuel stabilizer on hand if you plan to have your fuel around for longer.

If your gas does go bad, it is possible to use it so long as it’s mixed with a healthy amount of fresh gas.

Happy pumping!

Categories: Fuel Tips


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