Does gasoline degrade when it has been in a gas tank for, say, more than a year? I have an old Ford F-250 with a reserve gas tank and I have had nearly a full tank in reserve that hasn’t been used for more than a year now. Can that gas go bad? Is it still safe to use? Do I need to add something to that reserve tank before I switch over to it?

Richard B. Ellenberger, Burien, Wash.

Gasoline does degrade with time and can become unusable, depending on conditions and the vehicle in question. The lighter elements of the fuel evaporate, leaving a gummy, less volatile product. Your pickup’s reserve-tank gas began to go stale within the first two months of storage, and is now quite degraded.

Old gas can cause pinging, power loss and difficulty starting, as well as a tendency to gum up the tank, lines, pump, filter and carburetor (or fuel injectors, if so equipped). If one anticipates storage of a vehicle or fuel container for more than a couple of months, adding a container of Sta-bil or similar stabilizer/conditioner to a full tank (having as much fuel as possible avoids water condensation) can help preserve the gas.

What can be done with gas that already is degraded? This is a problem, as old gas is difficult, if not impossible, to properly dispose of. The best solution is to siphon as much of the old gas from the tank as possible, store it, then mix a few gallons back in with each fresh fill-up, to get rid of it. Since most fuel tanks don’t have a drain plug, siphoning or pumping the old gas will be necessary.

If siphoning, be sure to use a squeeze bulb or other means – not your mouth – to begin the process. Filtering the gas through a coffee filter as it enters the storage container is a good idea, as some particulate matter might be present.

Few, if any, will want to follow the above procedure. Storing that much old gas is problematic and potentially dangerous. Here’s an alternative that should work in your case because your truck has two fuel tanks: Siphon out 5 gallons and refill the tank with premium (high-octane) gas in addition to a container of octane booster (available at an auto-parts stores). Your reserve tank can then be used under infrequent, but optimum conditions: after start-up, and during morning, around-town and leisurely driving. High-temperature, high-load and at-highway-speed driving isn’t a good idea, as the engine might ping badly, with potentially damaging results. After you’ve run the reserve tank down, your 5 gallons of stored stale gas can be metered back in with fresh gas, a few gallons per fill-up.

I should add that modern fuel-injected vehicles are far more fussy about fuel quality and cleanliness than your pickup, even though their well-sealed fuel tanks might better preserve the gasoline. In this case, stale gas would be better consumed by the lawn mower, or given to a thrifty and potentially forgiving neighbor with a carbureted vehicle.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190.

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AP-NY-04-29-05 0622EDT