What To Do With Old Gas: Can You Still Use It?

 All petroleum fuels have a limited storage life. Over time, they gradually degrade because of chemical reactions taking place within the fuel itself. Those reactions are inevitable, but they're accelerated by real-world conditions like exposure to air, water, heat, and certain metals. The result is fuel that slowly changes in ways that can eventually affect both its performance and its ability to protect your engine. 

These changes don't happen overnight, and you can't watch them occurring in real time. What you eventually see are the results: gasoline that has darkened, formed gums and varnishes, or, in the case of ethanol-blended fuel, undergone phase separation. Diesel fuel experiences many of the same aging processes, although it doesn't have the phase-separation issue associated with ethanol.

When it comes to dealing with "old gas," this is primarily a consumer problem. Most businesses don't store gasoline for extended periods of time. If commercial fuel sits in storage for months or years, it's usually diesel. Consumers, on the other hand, often buy gasoline for lawn equipment, generators, motorcycles, boats, or other seasonal equipment, store what's left in a fuel can, and then rediscover it months later wondering whether it's still usable.

Can you still use old gas?

Sometimes—it depends entirely on how far the fuel has degraded. If the gasoline contains ethanol (as most gasoline does), the first thing to check is whether it has phase-separated. Fuel that wasn't treated before storage—or that has simply been sitting long enough—can eventually separate into two layers: a water-and-alcohol layer on the bottom and gasoline floating on top.

Once gasoline has fully phase-separated, it's generally considered beyond practical recovery. Fuel that has simply darkened but remains one uniform layer is a different story. In many cases, it can still be used successfully if it's properly treated before it's burned. The rest of this article explains how to tell the difference.

Why does gasoline go bad in storage?

Gasoline begins aging the moment it leaves the refinery. Over time, exposure to air, heat, moisture, and certain metals speeds up the chemical reactions that gradually change the fuel. The lighter, most volatile components evaporate first, which is why old gasoline often smells stale and becomes harder to ignite. At the same time, heavier fuel molecules react with oxygen and begin linking together into gums and varnishes.

Ethanol-blended gasoline introduces another challenge. Ethanol naturally absorbs moisture from the atmosphere. If enough water accumulates, the alcohol-water mixture separates from the gasoline and settles to the bottom of the container. This process, known as phase separation, is the most common reason stored ethanol-blended gasoline becomes unusable.

How can you tell if old gas has gone bad?

There are two primary things to look for: separation and color. Phase-separated gasoline will have two distinct layers in the container—a cloudy layer of water and alcohol on the bottom with gasoline floating above it. If you see that, the fuel has failed and shouldn't be used in an engine.

If the gasoline remains one uniform layer but has noticeably darkened compared to fresh fuel, it has undergone oxidation and formed gums and varnishes. While fuel in this condition is no longer ideal, it can often still be used successfully if it's treated before use. Without treatment, those heavier compounds can leave deposits in injectors, carburetors, intake valves, and combustion chambers.

Can a fuel additive fix phase-separated gas?

For all practical purposes, no. Once gasoline has fully phase-separated, there's very little that can be done to restore it economically. Professional fuel service companies sometimes have access to specialized products and procedures that can recover fuel under certain conditions, but those methods require equipment and expertise that aren't practical for most consumers.

Some fuel additives claim to "restore" phase-separated gasoline. In reality, they're generally referring to fuel that's only beginning to absorb water—not gasoline that has already separated into distinct layers. For the kind of old gasoline most people discover in a forgotten fuel can, there's usually no practical way to return it to its original condition.

What can you do with darkened gas that hasn't separated?

In many cases, you can still use it—provided you treat it before putting it into an engine. Darkened gasoline has already undergone chemical changes that produce gums and varnishes, and those materials can create deposits throughout the fuel system as they're burned.

A quality gasoline fuel additive helps address many of the issues associated with aging fuel. Depending on the formulation, it may help control deposits, improve combustion, support fuel stability, and provide additional protection against the effects of ethanol and moisture. Properly treated, gasoline that has aged but hasn't phase-separated can often be used successfully instead of being discarded.

Which detergents work best for old gas—PIB or PEA?

Both are proven gasoline detergent chemistries. PIB (polyisobutylene) detergents and PEA (polyether amine) detergents are both effective at helping control deposits that can build up in gasoline engines, including engines running today's ethanol-blended fuels.

But detergency isn't the only thing that matters when you're dealing with older gasoline. Depending on the condition of the fuel, you may also benefit from additives that help support fuel stability, protect against the effects of ethanol and moisture, reduce corrosion, or provide a combination of these benefits. The best choice depends on the condition of the fuel and the type of protection you're looking for.

Whether you're treating gasoline before long-term storage or preparing older gasoline for use, choosing a quality fuel additive formulated specifically for gasoline and ethanol blends gives you the best chance of avoiding fuel-related problems.