How Gasoline Goes Bad

Gasoline Goes Bad—What To Do To Prevent Stale Gas Problems in Your Small Engines

If there’s one thing I’ve learned after decades of working on engines, it’s this: keeping gas in your small engine for long periods is the best way to create starting problems. Many people have heard that gas can go bad, but few understand exactly how it happens or what it does to your engine. I’ll cover all of that in this article. I’ll also give you some tips on how to use fuel stabilizer to extend the life of gas and prevent no-start problems in your small engines.

What’s Actually in Gasoline?

Gasoline isn’t just one compound—it’s a complex blend of up to 150 different ingredients. There’s no universal recipe. Each refinery creates its own blend based on several factors: the season, local emission rules, the refinery’s equipment, and current prices of components. But even with all that variation, most gasolines share a common group of base ingredients:

• Paraffins (Alkanes) – 30–70%
Straight or branched hydrocarbons like hexane and isooctane. These provide the bulk of the fuel’s energy.
• Isoparaffins (Branched Alkanes)
Like isopentane and isooctane, these improve volatility and reduce knocking.
• Olefins (Alkenes) – 5–20%
Compounds like pentene and hexene. These are highly reactive and tend to oxidize, forming gums and varnish—the stuff that clogs carburetors and fuel injectors.
• Naphthenes (Cycloalkanes) – 5–10%
Cyclopentane and cyclohexane, which add to the energy content.
• Aromatics – 20–45%
Benzene, toluene, and xylene. These burn well and help prevent knock, but they’re also toxic and slow to evaporate.

Then Come The Performance Additives

Each gasoline brand adds their own additive package to the raw gasoline at the distribution terminal.
These aren’t just to boost performance—they help with storage, emissions, and protection. Some key additives include:

Detergents – Clean fuel injectors and intake valves
Anti-knock agents – Isooctane, ethanol, and aromatics to prevent engine knock
Oxygenates – Like ethanol (up to 10% in E10), which help reduce emissions
Stabilizers and antioxidants – Slow down fuel breakdown during storage
Corrosion inhibitors – Protect metal parts in the fuel system
Demulsifiers – Help separate water from fuel to prevent phase separation
Valve seat recession protectors – Especially for older engines
Deposit control agents – Reduce carbon buildup
Friction modifiers – Improve fuel economy
Conductivity improvers – Prevent static charge buildup in fuel systems

How Gasoline Degrades In Your Gas Can and In Your Tank

Evaporation – The most volatile components in gasoline evaporate when you fill your gas can, refuel your tank, or let gas sit unused. Even when stored, fuel continues to lose these light compounds over time—especially due to vapor permeation (see below). The more that evaporates, the less volatile and less effective your remaining fuel becomes.

Oxidation – When gasoline is exposed to air, oxygen reacts with its more chemically unstable components, especially the olefins containing reactive double bonds. These oxidation reactions produce peroxides, alcohols, ketones, and organic acids. Heat, sunlight, and trace metals like copper or iron in the fuel system speed up the process by acting as catalysts.

As oxidation progresses, these byproducts begin to polymerize into gums—sticky, tar-like substances that damage your fuel system. These non-volatile residues can:

• Clog the carburetor main and idle jets, prevent fuel flow to the engine.
• Clog the fuel inlet needle valve causing it to stick fully open or fully closed
• Form varnish on internal components

Gum formation is the most visible sign that gasoline has gone bad. These long-chain molecules no longer combust and serve only to create blockages, deposits, and no-start issues in small engines.

Moisture and Phase Separation – Gasoline can absorb moisture from the air over time, especially when it contains ethanol. Ethanol is hygroscopic—it pulls water from the atmosphere. In fuels like E10 (10% ethanol), up to 3.8 teaspoons of water per gallon can be absorbed before reaching saturation. Once that limit is exceeded, phase separation occurs.

During phase separation, the ethanol and water mixture detaches from the gasoline and sinks to the bottom of the tank. This water-saturated ethanol is what your engine pulls in first—leading to hard starts, poor performance, or complete no-start conditions. It can also damage carburetors and other fuel system components.

Water contamination in fuel can lead to:

• Hard starts or no-start
• Corrosion throughout the fuel system
• Incomplete combustion
• Engine hesitation or misfires

How Air and Moisture Infiltration, and Evaporation Occur in Your Gas Can and Gas Tank

Vented Gas Tanks Breathe

The gas tanks in small engines are vented to the atmosphere. As temperatures rise, the fuel heats and expands, pushing vapors out of the vent, leaving the less volatile components behind. As temperatures fall, the fuel contracts, pulling in outside air and moisture.

Volatile Components Escape Right Through Plastic Gas Cans

Even if your gas can is sealed, vapors can still escape, especially through plastic gas cans and gas tanks. Most portable gas containers and many small engine gas tanks are made from high-density polyethylene (HDPE).

• Components like n-butane, isobutane, pentane, benzene, toluene, xylene (BTX), and ethanol permeate through the plastic and are lost to the atmosphere.
• Heat: Higher temperatures increase permeation..
• Thin Walls: Thinner plastic walls allow vapors to pass through more easily.
• Age of the Plastic: As the plastic ages, it can develop microcracks or lose its density, making it
even more permeable. Old gas cans (pre-2009 in the U.S.) leak much more vapor, even if they seem perfectly intact.

Because of environmental concerns, volatile organic compounds (VOCs) contribute to smog and air pollution, regulations (like California’s CARB standards and EPA rules) now require low-permeation gas cans.

Low Permeation Gas Cans:
• Use special multi-layer plastics (like a thin barrier of ethylene vinyl alcohol, EVOH).
• Or coat the inside of the can with a chemical-resistant layer. This reduces vapor escape to a maximum of around 0.3 grams per gallon per day.

Non-CARB compliant versus CARB compliant gas cans

What Gum And Varnish Look Like

The carburetor float bowl shown on the right is coated with sticky

Gum and varnish in the carburetor bowl

gum and varnish from the oxidized gasoline. The gum and varnish clog the main and idle jets in the carburetor, causing no-starts, rough idling, and surging. The gum can also clog the needle valve, blocking fuel from entering the carburetor bowl. like aldehydes and organic acids) start to polymerize into gums—sticky, tar-like substances

Gum and varnish on the needle valve

What Water and Phase Separation Do to Carburetors

Water in gasoline causes additional oxidation and corrosion. The images here show a fully corroded carburetor.

The corrosion clogs the main and idle jets and causes the needle valve to stick.

How to Slow Fuel Degradation

Over the years, I’ve developed a simple set of habits that keep my equipment running strong year after year. Here’s how I prevent old gas from turning into a costly repair:

1. Buy Ethanol-Free Gasoline—Ethanol-free gas eliminates the problem of phase separation. You still have to be concerned about evaporation, oxidation, and moisture contamination, but at least you don’t have to worry about the alcohol separating from the gas.

2. Invest in a Metal Gas Can— Storing gas in a metal gas can eliminates the problem of losing the most volatile components to permeation.

3. Add Fuel Stabilizer at the Pump – Fuel stabilizer isn’t a magic fix for stale gas—it only works if you add it to fresh fuel. If I know the gas will sit unused for more than 30 days, I treat it right away—ideally, the same day I buy it. I use a high-quality stabilizer like K-100 or Star Tron. These products help slow oxidation, reduce gum and varnish formation, and prevent ethanol blends from separating. But timing is everything: once fuel starts to degrade, stabilizer won’t reverse the damage. It must be added before the fuel begins to break down.

4. Rotate Stored Gasoline— I keep track of when I fill my fuel cans. Anything older than three months goes into the car or truck, where the larger fuel system and fuel injectors can handle it better than a small engine. I never try to run a mower, snowblower, or generator on old gas if it’s more than 90 days old, even if it was stabilized.

5. Drain Fuel from Seasonal Equipment— When I put away my mower in the fall or the snowblower in the spring, I drain the tank and run the machine dry. I also empty the carburetor bowl if possible. Leaving even a few ounces of old gas in the carburetor over the off-season is a recipe for varnish and corrosion.

6. Store Gas Properly— Gasoline belongs in a sealed, approved container stored in a cool, shaded area. I avoid storing gas cans in direct sunlight or anywhere with big temperature swings, which speeds up evaporation and moisture absorption.

7. Buy Only What I’ll Use Soon— I don’t stockpile gasoline unless there’s a hurricane on the way. It’s tempting to fill multiple cans when prices are low, but unless I’m going to burn through it within a couple of weeks, that fuel will likely turn to stale gas and cause more problems than savings.

©, 2025 Rick Muscoplat