Gasoline is generally considered old after three to six months, depending on the type. Ethanol-blended fuel (the E10 or E15 you buy at most pumps) has the shortest usable life, typically degrading within one to three months as the ethanol absorbs moisture from the air. Pure, ethanol-free gasoline stored in a clean, sealed container away from heat and sunlight can last six months to a year before breaking down significantly.
Why Gasoline Doesn’t Last
Gasoline is a mixture of dozens of chemical compounds, and the lighter, more volatile ones start evaporating almost immediately once the fuel is exposed to air. This process, called weathering, shifts the balance of the fuel’s composition. The light compounds that ignite easily disappear first, leaving behind heavier, harder-to-burn components. Research on gasoline weathering shows this change is dramatic in the first six hours of exposure to open air, with a sharp drop in the volatile compounds that make gasoline effective as a fuel.
Oxidation compounds the problem. When gasoline reacts with oxygen over weeks and months, it forms gummy, sticky residues. These residues eventually harden into varnish-like deposits. The result is fuel that burns poorly and leaves behind material that can physically damage your engine and fuel system.
How to Tell If Your Gas Has Gone Bad
Fresh gasoline has a clear, slightly yellowish color. As it ages, it darkens noticeably. If you have gas stored in a can, pour a small amount into a clear glass or plastic container and compare it to a sample of fresh fuel. The difference is usually obvious.
Smell is the other reliable indicator. Old gasoline develops a sour, acrid odor that’s distinctly different from the sharp chemical smell of fresh fuel. You’ll typically notice it the moment you remove the cap from a gas can, without needing to lean in close. If it smells like something other than normal gasoline, it’s likely too degraded to use safely.
What Old Gas Does to Your Engine
Running old gasoline through an engine causes problems that range from annoying to expensive. Because the chemical composition has changed, the fuel doesn’t combust the way your engine expects. This shows up as hesitation or sputtering when you try to accelerate, engine misfires, and unexpected stalling while driving. Your car may be hard to start or idle roughly.
The longer-term damage comes from varnish and gum deposits. These sticky residues build up inside fuel injectors, carburetors, fuel lines, and filters. Varnish can clog fuel filters, restrict flow through narrow passages, and cause fuel injectors to stick. Clearing these deposits often requires professional cleaning or replacement of affected parts, which gets costly quickly. A lawnmower or generator that sat all winter with fuel in the tank is the classic example: come spring, the carburetor is gummed up and the engine won’t start.
Storage Conditions That Matter
How quickly gasoline degrades depends heavily on how it’s stored. Three factors accelerate the breakdown: heat, air exposure, and moisture. A sealed, airtight container in a cool, dry location gives you the longest shelf life. A vented gas can sitting in a hot garage during summer will degrade fuel much faster than the general timelines suggest.
Temperature swings are particularly damaging because they cause condensation inside the container. That moisture mixes into the fuel and, with ethanol-blended gas, gets absorbed directly into the ethanol. This is called phase separation, and once it happens, the fuel is essentially ruined. No amount of mixing will restore it.
For comparison, diesel fuel stored in cool, dry conditions lasts roughly six to twelve months, about twice as long as standard gasoline. Propane, because it’s a simple gas stored under pressure in sealed tanks, has an essentially unlimited shelf life.
What to Do With Old Gasoline
If you have a small amount of old gas (a gallon or less) that hasn’t gone completely dark or sour, you can dilute it by adding it to a nearly full tank of fresh gasoline in your car. The fresh fuel compensates for the degraded stuff, and your engine can typically handle the mix without noticeable issues.
Gasoline that’s clearly discolored, smells sour, or has visible sediment should not go into any engine. It also should never be poured down a drain, onto the ground, or into the trash. Old gasoline is classified as household hazardous waste. Most counties and municipalities operate hazardous waste collection sites or periodic drop-off events where you can bring old fuel, motor oil, and similar automotive fluids for safe disposal at no charge. Your local government’s website will list the nearest option.
Keeping Fuel Fresh Longer
If you regularly store gasoline for equipment like mowers, snow blowers, or generators, a few steps make a real difference. Use a tightly sealed container designed for fuel storage. Keep it in a cool, shaded spot. Buy only as much as you’ll use within a month or two.
Fuel stabilizer additives are widely available and effective. Added to fresh gasoline at the time of purchase, they slow oxidation and can extend shelf life to 12 months or longer. This is especially useful for seasonal equipment. If you’re putting a mower away for winter or a boat into dry dock, running stabilized fuel through the engine before storage protects the fuel system from gum and varnish buildup during the off-season.