How to Know If Your Car Has a Bad Tank of Gas

Key Points

• A bad tank of gas typically has water and/or debris in it.
• Bad gas can cause an engine to run poorly.
• Other engine issues can present the same symptoms as bad gas.

What might be a bad tank of gas can be due to a variety of issues, and it can prevent a car’s engine from running as well as it should.

Related: If Premium Gas Is Recommended for My Car, Will Using Regular Void the Warranty, Ruin the Engine?

What Causes a Bad Tank of Gas?

• Takeaway: A bad tank of fuel can be due to water or debris in the gas, a stronger mix of water/ethanol, lower-grade gas, a loss of volatility or degradation.

While relatively rare, gasoline can come from the gas station’s pump with water or debris such as rust already in it. Either will usually settle to the bottom of your car’s fuel tank, which is where the fuel pick-up is normally located. This allows the fuel pump to suck up the water and debris and send them to the engine. Your engine doesn’t run well when water is mixed with the gas, and the debris can clog your fuel filters or fuel injectors, starving the engine of fuel.

Another possibility is that ethanol (a grain alcohol made largely from corn that constitutes 10% of most gas nowadays) has combined with water and separated out of the gas. This combination likewise will settle down to the bottom of the station’s tank. If that tank is nearly empty when you fill up yours, you can get a stronger dose of water/ethanol mix than your car can handle, making it run poorly.

It can also be the result of the gas station putting lower-octane regular-grade gas into the high-octane premium-grade fuel tank, which would only be an issue if your car requires premium-grade fuel. Cars that only recommend premium will usually run all right on regular, though you might not have quite as much power.

One indicator of all of the above is if the car suddenly starts running poorly shortly after you fill it with gas, particularly if your tank was nearly empty prior to refueling.

Note that many of these problems can also occur in your car’s fuel tank. Water and ethanol can separate out due to condensation in the tank, particularly in cold weather with a low fuel level, and debris can come from a rusting fuel line. It’s also possible you accidentally filled the tank of your premium-required car with regular-grade gas.

Allowing a car to sit for a while can also cause gasoline to lose its volatility and degrade over time. In some cases, this can occur after only about three to six months, though gas stored in an airtight container will usually last longer. In addition, you never know how long the gas you bought was in storage at the refinery or in the gas station’s tank before you pumped it into your car. This can be of particular concern if the car sits over the summer or winter or doesn’t accumulate many miles.

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Checking for Bad Gas

• Takeaway: Determining if gas is bad is possible at home, but it’s a job best left to a shop.

Unfortunately, it’s difficult to determine that bad gas is what’s causing your car to run poorly, as it produces many of the same symptoms as several more common problems. Hard starting (or not starting at all), a rough idle, sputtering and poor acceleration can all be caused by bad gas, as well as any number of other issues.

Worse, while checking for faulty parts can be fairly straightforward, checking for bad gas is not as simple as it may sound. The hardest part is getting a sample of the gas to inspect.

In the good ol’ days, you could just siphon some gas out of the tank through the fuel-filler neck. But most modern cars have restrictions in the neck to prevent that, as people were stealing gas by siphoning it out (blame the gas crises in the 1970s). Today, the fuel line has to be disconnected and the fuel pump activated to collect any amount of gas in a clear glass container.

If you plan to do this yourself, there are a few points to keep in mind. A service manual or internet search can probably tell you where you can disconnect the fuel line and whether you’ll need special tools to do so. But with fuel-injected cars, you’ll first need to disconnect the battery ground cable (to avoid electrical sparks) and relieve the fuel pressure. Some cars have a pressure-relief valve in the fuel line near the engine, but you can also pull the fuel-pump relay or fuel-pump fuse and crank the engine to relieve the pressure. Then you can disconnect the fuel line and turn on the ignition to pump some gas into a clear glass container.

Once that’s done, it’s just a matter of a short wait.

Good gas is usually clear and maybe slightly gold colored, while old age and most kinds of contamination will turn the gasoline cloudy and/or darker gold. Water will quickly settle to the bottom, forming a glob that can be seen by tilting the jar at an angle. (You have to look closely, but it’s a different “clear” than the gasoline.) Ditto for ethanol that has separated out of the gas. 

If you can determine that any running problems might be caused by bad gas, the gas must be pumped out of the tank. This is hard to do and is best left to a mechanic. Given that, it’s best to leave the whole job of determining if your gas is bad to a mechanic, as well.

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