Is The Gas In Your Car Too Old?

By Eric Peters, Automotive Columnist

Everything deteriorates over time — including gasoline. Even if it’s stored properly. It’s as inevitable as graying hair and creaky joints.

Here’s why:

Gasoline is a highly refined product brewed to a certain chemical composition with very specific characteristics, including, for example, 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.

But the most highly volatile components in gasoline also tend to evaporate over time. As they do, the fuel’s volatility 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. It might be hard to start; it might cough and stall. Power — and economy — will likely go down.

The good news is the problem should cure itself — once the old gas has been consumed and the tank’s topped off with fresh fuel.

The next problem is oxidation — and it’s more serious.

Hydrocarbons in the gas react with oxygen to produce new compounds that eventually change the chemical composition of the fuel, leading to gum and varnish deposits in the fuel system. These deposits and impurities can coat surfaces, clog up fuel lines and filters, as well the small orifices in a carburetor (jets) and the even smaller orifices in a modern car’s fuel injectors.

Removing these deposits can be a major job — and your vehicle may not run at all (or run very poorly) until they are removed.

Lastly, there’s water contamination.

Condensation from heat cycling can form inside your vehicle’s gas tank and fuel lines, especially if the tank is not completely full.

Water contamination is also a problem at gas stations with light traffic, for the same reason. The underground storage tanks experience increases and decreases in temperature — and 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.

Another problem is ethanol alcohol, which now comprises as much as 10 percent of all “gasoline” sold in this country. Alcohol likes to draw moisture out of the surrounding air. And water does not work too well as a fuel in an internal combustion engine.

Water contamination can cause hard starting and rough running until the excess H20 is purged from the system. More seriously, it can also contribute to internal rusting of the fuel lines and tank. Scale and small particles breaking loose and circulating in the fuel can create a true nightmare — sometimes even requiring the replacement of the lines and tank at considerable expense.

How to identify bad gas?

One way is to look at it. Fresh gas is almost translucent and the color of light beer. Oxidized fuel often turns darker over time — and may even smell sour. You can check stored fuel by pouring some into a clear glass container and comparing it side-by-side with known fresh fuel. If your old sample looks noticeably darker than the fresh fuel it’s likely the fuel’s gone bad.

How long does it take for gas to go bad?

That depends on a number of factors. If the gas was very fresh when you bought it (recently refined/not stored in the gas station’s tanks for months before you purchased it) and you store it properly (see below) it might stay fresh for as long as a year or more.

But there’s virtually no way to know how fresh the fuel you just bought actually is — and most people don’t store their fuel for optimum shelf life — so the real-world longevity is much less, perhaps just a few months.

It’s therefore prudent to assume the fuel isn’t fresh — and to take steps to preserve its useful shelf-life as long as you reasonably can.

Here’s how:

  • Try to buy only as much fuel as you’ll actually use within a month or so; if you can do this, it’s a near-guarantee that the fuel you have will always be fresh.
  • Keep your vehicle’s gas tank topped off to reduce water contamination.
  • Be sure all jugs/containers used to store fuel are topped off and sealed tightly to limit exposure to oxygen, which increases chemical decomposition of the fuel.
  • If you have an antique or classic car that’s not driven often, be sure to top off the tank after every time you do drive it (same goes for motorcycles).
  • Add fuel stabilizer (such as Sta-Bil, available at auto parts stores, etc.) in the appropriate ratio to fuel that will sit for more than two or three months — as in the gas tank of a lightly used vehicle, lawn equipment during the off-season or fuel storage containers.

Fuel system stabilizer is not a cure-all, however — and it doesn’t make gas last forever. It must be mixed with fresh gas before the vehicle is stored — not added to already old gas. And while it can slow down the oxidation process and keep gas fresh for as long as 12-15 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.


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