How to Tell If a Car Got Bad Gas

Cars with bad gas display some very obvious symptoms. Preventing the problem begins with choosing the correct octane rating when fueling --- an owner's manual, a manufacturer or a qualified repair person can help identify proper octane ratings for each vehicle. Gas can also become contaminated with dirt and rust or lack the necessary amount of additives from the manufacturer. Inexpensive gas is sometimes watered down, causing the car to respond poorly when accelerating or idling. A few tests can indicate that bad gas may be the cause of a car's poor performance.

• 1

  Start the car and let it warm up. Listen to the engine as the car idles; "knocking" and "pinging" sounds are indications that the gas does not have enough octane in it. What is labeled as high-performing octane gas can sometimes be mixed with lower rated gas, causing the octane rating to drop and not be adequate for the car.

• 2

  Drive the car and perform several tests on it --- it's best to do this when traffic is light. Let the car idle at a street light and then watch what happens when you accelerate. Cars that hesitate or stall could have bad gas in the tank. Rough idling while sitting at a light or stop sign is another indicator.

• 3

  Drive the car down a street. As you accelerate, notice how the car responds. Hesitation, slow acceleration and poor response can mean bad gas. A backfiring vehicle is another sign the gas is of poor quality, especially if the car begins backfiring right after gas has been added.