Info on Gas

[Wathen1955]

I've always wondered if gas goes bad if it sits for a long period of time. I have a small 3 gallon plastic container that I store gas for my lawn mower. I only fill it once a year and never had a problem. The car on the other hand, it a different animal. I found some interesting facts on gas:

Does Gas Go Bad?
Does gasoline really go "bad" if you leave it unused for a period of time? Some people are convinced this is just another urban legend, and that people who worry about "old gas" and spend money on fuel stabilizer are wasting psychic energy as well as cash.

But in fact, gasoline can degrade over time. That can lead to a number of problems, ranging from hard starting, to rough running, to no starting at all.

Here's Why

Unlike crude oil, gasoline is a highly refined product brewed to a certain chemical composition with very specific characteristics. One characteristic of gas is volatility, a term used to describe how easily and under what conditions the gas vaporizes so it can be efficiently burned in your car's engine.

The most highly volatile components in gasoline also tend to evaporate over time. As they do, the remaining fuel's volatility and ability to combust properly degrades. The less volatile the fuel, the less effectively it burns in your engine. The result is diminished engine performance. Your engine may still start and run, but it probably won't run as well.

The good news is, once the old gas has been consumed and the tank is topped off with fresh fuel, the problem should cure itself. Evaporation of volatile compounds can be limited by making sure the gas cap is secured tightly. For the same reason, be sure all portable gas containers are sealed tightly as well.

A More Serious Problem: Oxidation

Hydrocarbons in the gas react with oxygen to produce new compounds that eventually change the chemical composition of the fuel. This leads to gum and varnish deposits in the fuel system.

These deposits and impurities can clog up gas lines and filters, as well the small orifices in a carburetor and the even smaller orifices in a fuel injector. Removing these deposits can be expensive and your vehicle may not run at all or run very poorly until they are removed.

Water Contamination

Condensation can form inside your gas tank and lines from heat cycling. Fuels such as E85, which have a high concentration of ethanol alcohol, may be even more susceptible to water contamination, as ethanol likes to draw moisture out of the surrounding air.

Water contamination can be a problem at gas stations with light traffic due to a slightly different kind of heat cycling. The underground storage tanks experience increases and decreases in temperature. This can cause moisture to form and contaminate the fuel. When you fill up at such a station, you're pumping in the water along with the gas. Such low-traffic stations may also have other contaminants in their underground storage tanks, such as rust. They are best avoided when possible.

Water, of course, does not work too well as a fuel in an internal combustion engine.

It will cause hard starting and rough running until it's purged from the system. It can also contribute to internal rusting of the gas lines and tank. The resultant scale and small particles can create a true nightmare, sometimes requiring the replacement of the gas lines and tank at considerable expense.

You can reduce the chances of water contamination by keeping your car's gas tank as close to full as possible, especially if the vehicle is going to be left idle for an extended period.

How Do You Identify Bad Gas?

One way is to eyeball it. Oxidized fuel often turns darker over time and may even smell sour. You can check stored gasoline by pouring some into a clear glass container and comparing it side-by-side with known fresh gasoline. If your old sample looks noticeably darker than the fresh gas, you have strong evidence the gas has gone bad.

How Long Does it Take for Gas to Go Bad?

That depends on a number of factors. For one, it's hard to know how old the gas you just bought actually is. It may be fresh from the refinery, or it may be a month old already by the time you top off your tank. Some gasoline is mixed with better or more oxidation inhibitors than others.

It's a good rule of thumb to avoid leaving gas in your tank or a storage container for more than a coupe of months, if you can avoid it.

And if You Can't?

If you know gas will sit in your tank or a storage container for a couple months, then it's a wise move to buy some fuel system stabilizer and mix it in with the gasoline. Do it before you put the vehicle into long-term storage or before leaving your lawn equipment fuel containers sitting for the winter. The stabilizer helps prevent oxidation, the biggie that can turn gas into garbage that gunks up your system and leads to expensive repair work.

Using fuel system stabilizer for extended storage is preferable to draining the tank and leaving the system dry. This can cause rubber hoses, gaskets and seals to dry-rot and crack, possibly leading to leaks and even a fire. In addition, a dry system can expose the insides of metal fuel lines and your gas tank to air and moisture, which can lead to or accelerate the formation of rust.

Fuel system stabilizer is not a cure-all and it doesn't last forever. It must be mixed with fresh gas before the vehicle is stored, not added to already old gas. It can slow down the oxidation process and keep gas fresh for as long as 12 to15 months. If you're going to leave the vehicle parked for longer than that, you may want to drain the tank and refill with fresh fuel before returning the vehicle to service.

Does Gas Go Bad?

Then I went to Chevron:

Chevron gasoline can be stored for a year without deterioration when the storage conditions are good — a tightly closed container and moderate temperatures.

Most gasoline is used within a week or two of purchase. But there are occasions when gasoline is kept longer. Examples are gasoline stored to fuel small-engine equipment or to provide a backup supply for a vehicle. Also gasoline left in the tank of a boat stored for the winter or a snowmobile stored for the summer.

Chevron recommends that gasoline not be stored unnecessarily. A supply that won't be needed for several months should be used and replenished when the need reoccurs. Gasoline that is in good condition can be added to the fuel tank of a gasoline-fueled car or truck. (Don't dispose of gasoline-oil mixtures for two-stroke-cycle engines in this way.)

However, when storage is necessary, this bulletin describes how to do it properly and safely.

The term "container" is used collectively for storage cans and drums, storage tanks, and vehicle and equipment fuel tanks except when the discussion is specific to only one member of the class.

Chevron Products: Technical Bulletin: Longer-Term Storage of Gasoline

[Wathen1955]

Additional info:

More detailed info:

Volatility

Volatility describes a gasoline's tendency to form vapors. Liquid gasoline does not burn; only gasoline vapor burns. To start a cold engine, enough low boiling components ("light ends") must vaporize at the engine temperature to form a combustible vapor-air mixture.

This is one reason that the volatility of gasoline is tailored for the range of temperatures expected in the locality where is it sold. "Winter gasoline" has a higher volatility for easy starting in cold weather. "Summer gasoline" has a lower volatility because hydrocarbon vapor in the atmosphere contributes to smog formation.
Evaporation

The gasoline light ends needed for easy starting have the same tendency to vaporize in storage as they do in an engine. If the storage container is not tightly sealed, some of the light ends gradually will be lost. Too great a loss decreases the gasoline's ability to start an engine.

Evaporation of gasoline from a vented fuel tank or a can with a loose cap would be minimal if the temperature of the container were constant. But daily temperature changes cause the temperature of the container to cycle. The heating portion of the cycle raises the pressure of the gas (gasoline vapor and air) above the liquid gasoline which, in turn, drives some of the vapor-air mixture out of the container. The succeeding cooling cycle lowers the pressure of the gas, drawing fresh air into the container. Light ends evaporate from the liquid gasoline to saturate the new air. The daily repetition of this cycle gradually pumps light ends out of the container.

The cycle also brings air and water vapor into the container, especially during periods of high humidity. The oxygen in the air contributes to gum formation. (See Oxidation section.) And the water vapor, if it condenses during the cooling cycle, contaminates the gasoline with liquid water.

A larger volume of gas will be pumped in and out of the container when the air space above the liquid fuel is larger and when the daily temperature change is larger. Consequently, keeping the container almost full of gasoline and controlling the temperature fluctuations will minimize the loss of light ends, the exposure of the gasoline to air, and the contamination of the gasoline with water.
Oxidation

Except for any added oxygenates, gasoline is made up almost entirely of hydrocarbons–molecules constructed from the building blocks of elemental carbon and hydrogen. Hydrocarbons, as a class, are chemically stable molecules.

However, there are types of hydrocarbons (olefins and diolefins) that can combine slowly with the oxygen in the air ("oxidize") at ambient temperatures. The products of the reaction are larger molecules, collectively called "gum."

The gum-forming reactions become faster as the temperature of the gasoline increases. This is why this bulletin recommends controlling the temperature of stored gasoline.

Most gasolines contain negligible amounts of gum when they are manufactured, and most contain special chemicals ("stabilizers") to retard gum formation. It is the stabilizers that make it possible to store Chevron gasolines for a year when the conditions are good.
Soluble Gum

The gum formed by oxidation is usually soluble in gasoline. However, it remains behind as a sticky residue when the gasoline evaporates. Since gasoline begins to evaporate in the carburetor of a carbureted engine or in the injector of a fuel-injected engine, a gasoline containing soluble gum may leave a deposit on these parts and on the intake valves. These deposits will be in addition to the deposits normally formed by a gum-free gasoline — a formation triggered by the elevated engine temperatures.

Modern engines are designed to run best when vital engine parts are clean. Carburetor and fuel injector deposits can cause hesitation and stumbling on acceleration, lower fuel economy, lower power output, and higher emissions of hydrocarbons and carbon monoxide. Excessive intake valve deposits can cause many of the same performance problems, plus higher emissions of hydrocarbons, carbon monoxide, and nitrogen oxides.

Because the Federal Environmental Protection Agency recognizes that fuel system deposits increase emissions, they require all gasolines to contain a deposit-control additive. All deposit-control additives keep deposits from forming; the best ones clean up deposits formed by lower-quality gasolines.

If the gasoline contains a lot of soluble gum, the normal level of deposit-control additive may not be sufficient. This is why Chevron recommends treating a tank of gasoline with an extra dose of deposit-control additive if a vehicle displays driveability problems after being stored. (Note: Chevron does not recommend adding a deposit-control additive to the fuel for a two-stroke-cycle engine.) The gum-forming reactions become faster as the temperature of the gasoline increases.
Insoluble Gum

Severe oxidation of gasoline may produce insoluble, as well as soluble, gum. The insoluble gum will take the form of brown or black particles which float in the gasoline or settle to the bottom of the container.

When an engine is fueled with gasoline containing insoluble gum, the fuel filter will remove the gum. If the engine has an in-tank fuel pump, the screen on the pump's feed also may capture some of the gum. However, these devices can become plugged if the gasoline contains too much insoluble gum. This will cause the engine to lose power or stall because it is starved for fuel. Adding a deposit-control additive will not keep insoluble gum from plugging filters and screens.
Other Issues

Storage, including storage involving gum formation, does not change the bulk properties and most of the performance characteristics of gasoline (excluding the characteristics affected by the gum itself). For example, storage does not change a gasoline's anti-knock index (octane) or energy content. However, these properties will change if the storage is accompanied by evaporative loss. The evaporation of light ends decreases a gasoline's antiknock index and increases its energy content.

Federal and California reformulated gasolines will survive storage as well or better than conventional gasolines. The regulations require reformulated gasolines to have less light ends and less olefins (federal, later; California, now) than conventional gasolines. As explained above, it is the oxidation of olefins that leads to gum formation. Reformulated gasolines also contain oxygenates. The common oxygenates are stable molecules that do not form gums.

Technical Bulletin: Longer-Term Storage of Gasoline: Background Information

[mike100]

I've had to get a couple of cars running that had several year old gasoline before...basically you had to dispose of the gas. If the tank was full, it probably prevented rust from occurring, but still, it does a number on carburetors.

gas is usually ok for 6 months for sure and maybe more if the tank is well sealed.

[Sweet 90]

Some interesting info. Good work