Yes, the Gas in Your Car Can Go Bad — Here's How to Tell

Stephen Fogel
March 13, 2019

How often do you fill up your gas tank? Once a week? A couple times a month? Most drivers don’t have reason to worry about how long gasoline has been in their car, because it doesn’t sit for very long.

But gas can go bad. Maybe you have a car that you never drive — it sits for months at a time. Or maybe your wintertime fueling habits don’t include keeping your tank as full as possible. These scenarios and more can lead to a case of bad gas in your tank. 

What is bad gasoline?

Bad gas is usually the result of one of these two situations:

Chemical deterioration: The gas in your car is an extremely complex chemical, composed of hundreds of different compounds. That aroma that you sometimes smell when you fill up is the evaporation of the lighter-weight components of gasoline, and it occurs whenever the fuel reacts with oxygen.

If gasoline sits for too long, either at a gas station or in your gas tank, so many of these compounds can evaporate away that the octane rating of the gas drops to a point where it won’t burn efficiently in your engine. If left alone for months and months, the gas can eventually turn liquid gasoline into a thick, varnish-like material.

Water contamination: If water gets into the gas in your car, you can expect some serious problems. If this happens in the winter, condensation can create enough water in your tank to freeze the gas line solid. Water in your gas can also affect the way your engine runs, can cause corrosion in your fuel system, and can block the gas in your car from getting to your engine.

» MORE: Find a certified repair shop near you

How do you know if you have bad gas?

Not surprisingly, bad fuel will make your car run worse, if it runs at all. Here are the signs of bad gasoline in your tank:

  • Your car won’t start or starts hard
  • The engine idles roughly, sputters or stalls
  • You feel hesitation when accelerating or cruising at a steady speed
  • You hear pinging sounds from the engine, especially when you accelerate
  • Your check engine light comes on while any of this is happening

How long does it take for gas to go bad?

The gas in your car has a shelf life of several months. But there’s no way for you to know how old the gas in your car actually is. When did it leave the refinery? How long was it stored after it was refined? How long was it at the gas station before you purchased it?

The best course of action here is to use up the fuel in your tank by driving your car regularly. If you are storing extra gas in a gas can, these rules apply to that as well — use it as soon as possible, because it won’t last forever. 

What if your car has bad gas?

If you experience the symptoms listed above, call a mechanic, who can take a close look at the gas in your car and its fuel system, and troubleshoot the cause of your problem. Dealing with highly flammable gasoline and pressurized fuel systems is a job for a professional.

For large amounts of bad gas: If the gas in your car is old or contaminated with water, and it makes up all or most of what’s in your tank, the best solution is to remove all of the bad gas from your car. This may involve removing the entire contents of your gas tank, and then refilling it with fresh, uncontaminated fuel.

Depending on how bad the gas in your car is, your mechanic may be able to get away with simply siphoning the bad gas out. If the bad gas has left sediment and other contaminants in your tank, the mechanic may need to remove the tank, thoroughly clean it out, and reinstall it. Your fuel system could also need some work if it has been clogged or damaged by the bad gas in your car.

For smaller amounts of bad gas: If the symptoms of the bad gas started after you put less than half of a tank of gas in your car, you can try using a fuel additive or stabilizer that absorbs the water in your gas tank. You can also fill the tank with name-brand premium gas, which will blend with the bad gas and make it combust better in your engine. Refill your tank with more premium, whenever it gets down to the three-quarters mark. The good gas should eventually displace the bad gas in your car.

If you know where you got the bad gas in your car, don’t gas up there again.

An ounce of prevention is worth a gallon of cure

Here are a few tips that can protect you from dealing with bad gas in your car:

  • Get your gas at a station that is often busy; it will have the freshest fuel
  • Stick to Top Tier gas; the quality will be more consistent
  • Keep your tank full to reduce condensation and water buildup, especially in the winter
  • Be sure that your gas cap fits securely and seals tight

If you have to leave your car sitting idle for a few months, it’s a good idea to fill up the tank, add a fuel stabilizer, and drive it enough to circulate the stabilizer throughout your car’s fuel system before you park it. This should prevent the fuel from deteriorating over time and causing any related problems.

Stephen Fogel

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.

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