The short answer
Untreated gasoline can start causing storage problems in as little as 30 days.
Gasoline starts changing as soon as it is stored. In a generator, untreated fuel can begin causing problems in less than 30 days, and heat, humidity, an older gas-can fill, or a partly filled tank can shorten that window. Fresh fuel treated immediately with a storage stabilizer may last much longer—the STA-BIL product used here claims up to 24 months when used as directed—but your generator owner's manual should always control the storage plan.
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Watch the full video above for the old-fuel demonstration plus extension-cord, oil, backfeeding, and carbon-monoxide safety mistakes.
What changes in storage
Why Old Gas Causes Generator Problems
Gasoline oxidizes and loses the qualities that help an engine start and run correctly. At the same time, light fuel components evaporate and sticky deposits can form. In a small carburetor, those deposits can restrict tiny passages and leave a generator hard to start, surging, or running poorly.
Ethanol-blended fuel can also pull in moisture. Under the wrong conditions, water and fuel separation can contribute to corrosion and more starting problems. That is why a generator can look ready in the garage but fail when the power goes out.
Useful timeframes
How Long Is Generator Gas Good For?
Those are planning windows, not a promise that fuel is safe or usable on a particular day. Honda notes that regional fuel blends and storage conditions can cause deterioration in as little as 15 days. See Honda's generator fuel guidance and STA-BIL's product directions.
Fennell Tested approach
What We Do With Our Generator
If the fuel is questionable or we are putting the generator away for long-term storage, we would rather remove it than leave degraded gasoline sitting in the carburetor and fuel system. That is the practical approach shown in our generator video.
We do not treat stabilizer as a way to bring old gas back to life. If we cannot confidently say the fuel was fresh and treated correctly, we remove it using the generator manufacturer's procedure and start with known fresh fuel before we need the machine.
When storage is planned
Should You Use Fuel Stabilizer?
Fuel stabilizer makes sense when fresh gasoline will sit long enough for storage to become a concern and your generator manufacturer allows it. Add it to fresh fuel—not fuel that is already stale—and follow both the stabilizer label and the generator owner's manual. Many manuals also call for running the engine outdoors long enough to move treated fuel through the system.
STA-BIL Storage Fuel Stabilizer
Use only with fresh fuel and follow the product label and generator manufacturer's storage directions.
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Safe next steps
What If Old Gas Is Already in the Generator?
- Do not assume it is usable. If you do not know its age or condition, treat it as questionable.
- Let the generator cool completely. Work outdoors, away from flames, sparks, appliances, and anything that can ignite gasoline vapor.
- Follow the owner's manual. Use the exact fuel-removal or storage procedure for that model and collect fuel only in an approved, sealed gasoline container.
- Dispose of unusable fuel correctly. Contact your local household hazardous-waste program or fire department for local instructions. Never dump gasoline on the ground, into a drain, or into the trash.
- Get help if the generator still runs poorly. A carburetor or fuel system already affected by deposits may need professional service.
Between outages
How to Store a Generator Between Uses
- Fuel: Follow the manual's storage procedure; use fresh treated fuel only when the directions call for it, or drain the system as specified.
- Oil and maintenance: Check the oil level, service the air filter, and complete any scheduled oil or spark-plug maintenance.
- Battery, if equipped: Follow the manual for charging, disconnecting, or maintaining the starter battery.
- Storage location: Let the generator cool, clean it, and store it dry and protected from weather, fuel vapors, and ignition sources.
- Owner's manual: Keep it with the generator so the correct startup and storage steps are available when needed.
Watch: 5 Portable Generator Mistakes
Old fuel is only one mistake worth preventing.
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