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Can Bad Gas Cause Rough Idle? Is Bad Gasoline the Culprit Behind Engine Problems?

A gas can containing old gas.

If gasoline sits in a gas tank for too long, it may be contaminated with moisture and other contaminants, possibly leading to engine problems. But does it have damaging effects? Can bad gas cause rough idle?

Yes, bad gas can cause rough idle. Gas goes bad over time due to moisture contamination or contamination by other particulates, such as rust from the inside of the gas tank. Gas with water in it will not combust properly, and particles, like rust, may plug parts of the fuel system, such as the pump, filter, or injectors, all of which will cause an engine to idle roughly.

Let's look at what happens when you have bad gas in your vehicle.

Table Of Contents [show]

    A gas can containing old gas.
    A gas can containing old gas.

    Can Old Gas Cause Rough Idle?

    Yes, old gas can also cause rough idle.

    Old gas that has been stored for a long time separates from itself at a molecular level. As a result, its combustible molecules have either evaporated or are no longer distributed evenly enough to ensure proper combustion.

    Bad gas can also indirectly cause a rough idle by fouling the spark plugs.

    How Can You Tell If You Got Bad Gas In Your Vehicle?

    Indications of bad gas in your vehicle are acceleration issues, plugged fuel filter, a check engine light, and a poorly running engine.

    The following are the most common symptoms of bad gas in a car.

    1. Plugged Fuel Filter

    The fuel filter in your car is responsible for keeping sediment and debris out of the fuel system. All gas tanks have some sediment at the bottom, which the fuel filter captures.

    Usually, a fuel filter is replaced after 30,000 miles or two years, anything less, and you may have bad gas in your fuel tank.

    2. Acceleration Issues

    Gas quality has a direct impact on a vehicle's acceleration.

    Acceleration requires more fuel than maintaining a constant speed, so it exacerbates any engine problems caused by bad fuel.

    3. Rough Engine Idling Or Stalling

    Bad gas can cause an engine to stall or idle rough.

    When contaminated fuel is in your system, the combustion process is inefficient, leading to a rough idle or even engine stalling.

    4. Illuminated Check Engine Light

    The check light on your dashboard can indicate engine troubles caused by bad gas. However, since this indicator is ambiguous, you'll need a scanner to decode what the light is trying to tell you.

    Amazon has a multitude of OBD2 scanners at affordable prices like this one here (paid link).

    Fueling up a car with new fresh gas.
    Fueling up a car with new fresh gas.

    Is It OK To Mix Old Gas With New Gas?

    Ideally, you should siphon the old gas from the gas tank before adding new gas, but you can mix old gas with new gas if necessary.

    However, there are some things to keep in mind, such as the ratio between the old gas and the new gas, the age of the old gas, and whether or not you believe the old gas to be contaminated.

    In any gas, it won't hurt to add a gas treatment to the old gas and continue to top up with new gas every quarter tank to further dilute the old gas while you burn the mixture through your engine.

    Can Bad Gas Be Fixed With A Gas Treatment?

    If your rough idle is caused by bad gas, it could be fixed by a gas treatment. Ideally, gas treatment should be added to a near-empty gas tank, then mixed in by adding fresh gasoline.

    Can A Bad Gas Cap Cause Rough Idle?

    Yes, a bad gas cap can also cause rough idle.

    The gas cap's seal maintains the pressure in the fuel system. Therefore, if the gas cap is bad, it may lead to a check engine light, insufficient fuel system pressure, a source of idle problems in your car.

    Besides messing with the vehicle's fuel system pressure, a damaged gas cap can also allow dirt, moisture, and other impurities into the gas tank, which can lower the fuel delivery to the engine, making it idle roughly.

    It may take a tank or two of gas after replacing a bad gas cap before the vehicle's idle smooths out, and the check engine light resets.


    Gasoline can become contaminated with moisture or other particles, such as rust if it has been sitting around for too long.

    The contamination prevents the fuel from combusting properly, which may cause engine issues such as rough idling and stalling.

    Besides rough idling, some indications of bad gas include a check engine light, plugged fuel filter, and acceleration issues.

    A loose or damaged gas cap can also cause rough idling, so make sure your gas cap is in good condition and sealed properly.