Can you Leave old gas in a Generator or Should you Drain it?

The generator has been sitting in the back of the garage since the last outage, and there is still fuel in it. Half a tank, maybe more, and no memory of whether anything went in with it.

Whether you can still use that gas depends on more than its age. Was it treated before storage? Has water gotten into it? Has the gasoline phase-separated or deteriorated enough that it should come out? And what does the generator manufacturer say about storing fuel in the machine in the first place?

Untreated gasoline can begin degrading in as little as 30 days, and a generator carburetor is especially sensitive to the deposits aging fuel can leave behind. A visual inspection of the fuel can tell you whether obvious water, sediment, or phase separation has developed, but appearance alone cannot tell you everything about the condition of old gasoline.

Can you leave gas in a generator between uses?

For short periods between uses, fresh gasoline can remain in a generator. For longer storage, follow the procedure specified by the generator manufacturer. The important distinction is that untreated gasoline should not simply be left sitting in the fuel system for months.

Briggs & Stratton notes that gasoline can begin degrading in as little as 30 days and recommends either storing a portable generator with an empty fuel tank or adding stabilizer to clean, fresh gasoline and running the generator for several minutes to circulate the treated fuel through the system. Generac's current guidance is more conservative for long-term storage, recommending that portable generators stored for more than 30 days have the fuel drained and the remaining gasoline run out of the system.

The carburetor is particularly vulnerable to aging gasoline. Its fuel passages and jets are extremely small, and gasoline that oxidizes during storage can leave behind gum and varnish that restrict fuel flow. The carburetor bowl is especially susceptible because a small amount of gasoline can remain there and gradually evaporate, concentrating whatever the fuel leaves behind.

That is why the safest storage procedure is the one specified for your particular generator. Whether that means storing it empty or with properly treated fuel, the goal is the same: don't leave aging, untreated gasoline sitting in the carburetor for months.

Where does the water in generator fuel come from?

Water can get into generator fuel in several ways. Humid air can enter a vented tank as temperatures change, particularly when the generator is stored outdoors or in an unconditioned space. Water can also enter through contaminated fuel, poorly sealed storage cans, or direct exposure to rain and moisture.

Ethanol-blended gasoline adds another concern because ethanol has an affinity for water. Small amounts of water can remain dissolved in the ethanol-gasoline blend. If enough water accumulates for the fuel to exceed its water tolerance, the ethanol and water can separate from the gasoline and settle into a distinct lower layer. That is called phase separation.

Phase-separated fuel creates a serious problem for a generator because the fuel pickup is usually near the bottom of the tank—exactly where the water-and-ethanol-rich layer collects. At the same time, the gasoline remaining above it has lost some of the ethanol that contributed to its original octane rating. Once distinct phase separation has occurred, the fuel should generally be removed rather than trying to restore it with an additive.

Ethanol Defense is formulated with alcohol-free water-control chemistry designed to help manage moisture in ethanol-blended gasoline before phase separation occurs. The best time to address that water is before the fuel has separated into distinct layers.

Can you use old gas that has been sitting in a generator?

Sometimes, but age alone does not tell you whether gasoline is still usable. Fuel that has simply aged somewhat may still be usable, particularly when it remains clear and uniform and has no obvious water, sediment, or strong sour or varnish-like odor. Badly degraded or contaminated gasoline should be removed rather than trusted in a generator.

A clear glass container can help identify some of the most obvious problems. Let a small fuel sample settle and inspect it against a light or white background. Two distinct layers—particularly a cloudy lower layer—can indicate water contamination and phase separation. Sediment, heavy discoloration, or visible contamination are additional reasons to question the fuel.

A uniform appearance does not prove that old gasoline is still good. Gasoline can lose volatility and oxidize without separating into visible layers. If the fuel is only moderately aged and otherwise appears normal, diluting it substantially with fresh gasoline and treating the combined fuel may be an option. The older or more questionable the fuel is, however, the less sense it makes to depend on it in equipment that may be needed during an emergency.

If the fuel has phase-separated, contains substantial water or sediment, or smells strongly sour or varnish-like, remove it from the generator. Do not expect an additive to turn severely degraded gasoline back into fresh fuel. Dispose of unusable gasoline according to local hazardous-waste requirements.

Should you leave gas in a generator over winter or drain it?

There isn't one storage procedure that applies to every generator. Some manufacturers allow properly stabilized gasoline to remain in the fuel system during storage, while others recommend draining the generator completely for extended storage.

Generac currently recommends draining the fuel from portable generators that will be stored for more than 30 days. Its procedure includes draining the tank into an approved gasoline container and removing or consuming the fuel that remains in the carburetor so untreated gasoline is not left there during storage.

Briggs & Stratton allows two approaches for portable generators: store the machine with an empty fuel tank, or add stabilizer to clean, fresh gasoline and run the generator long enough to circulate the treated fuel through the fuel system.

That makes the owner's manual the deciding authority. If your generator's manufacturer calls for draining during long-term storage, drain it. If it allows storage with stabilized gasoline, start with fresh fuel, add the stabilizer before storage, and circulate the treated fuel through the system according to the manufacturer's instructions.

Whichever method your generator uses, don't simply park it for the winter with untreated gasoline sitting in the tank and carburetor.

How do you keep generator gas from going bad in storage?

The best time to protect generator gasoline is when the fuel is fresh. If you plan to store gasoline for later generator use, add an appropriate stabilizing treatment when you fill the container rather than waiting until the fuel has already been sitting for months.

For gasoline that will remain in a generator, follow the manufacturer's storage procedure. If the manufacturer allows stabilized fuel to remain in the machine, run the engine long enough to circulate treated gasoline throughout the fuel system. If the manufacturer calls for draining during long-term storage, remove the fuel and clear the carburetor according to its instructions.

Ethanol Defense is designed for ethanol-blended gasoline and combines fuel stabilizers with professional-strength, alcohol-free water-control chemistry. The stabilizers help slow the oxidation that occurs during storage, while the water-control chemistry helps manage moisture and reduce the risk of phase separation. Its detergent package also helps control deposits in the fuel system.

Reserve gasoline deserves the same attention. Store it in an approved, tightly sealed fuel container, keep it in a cool location away from ignition sources, and treat it while it is fresh. Rotate stored gasoline periodically rather than allowing the same fuel to remain untouched year after year. Using the oldest properly stored fuel first and replacing it with fresh, treated gasoline keeps your emergency supply from becoming a liability when you actually need the generator.