Symptoms of Bad Gas in a Car

by Jason MedinaUpdated October 25, 2017

A gasoline-powered engine needs a combination of both gasoline and air in order to start and run. Clean, high-quality gas is necessary to maximize engine operation. Bad gas can seriously disrupt normal engine operation and cause a variety of performance-related symptoms.

Engine Pinging

A major symptom of bad gas is engine pinging, or knocking, which typically occurs as the result of poor or incomplete engine combustion. Bad gasoline, which is a term normally used to refer to gasoline that has a high water content or insufficient octane, does not ignite and burn inside the engine cylinders as well as high quality gasoline. This typically causes engine pinging, especially under acceleration when improper engine combustion becomes more pronounced.

Engine Dieseling

Engine dieseling occurs when a vehicle's engine continues to run erratically, or diesel, after the engine has been turned off. This can occur for a variety of reasons, but bad gasoline is one of the main causes. Bad gas with a high water content frequently causes incomplete and abnormal engine combustion, including abnormal combustion that sometimes occurs for several seconds following engine shutoff.

Reduced Engine Power

Bad gas does not combust and burn as well as high quality gas. As a result, bad gas typically results in poor engine performance and reduced engine power. Proper engine combustion is necessary for maximal engine power and performance. Any disruptions to normal engine combustion can reduce engine power and negatively impede normal engine performance.

Engine Hesitation

An engine will often hesitate, or stumble, during acceleration due to the effects of bad gas. Engine hesitation often occurs as the result of incomplete engine combustion, which can be caused by bad gas, in addition to fuel delivery problems and spark plug misfires. Typically, bad gas that has insufficient octane, which reduces its combustibility, is the most likely type of gas to produce engine hesitation.


A vehicle engine that is hard to start can often times be a symptom of bad gas. Ignition is dependent on a steady flow of clean, high-quality gas into the engine cylinders, where the gas is ignited by the spark plugs, which in turn causes an engine to start. Bad gas that is low in octane or high in water content can prevent or seriously retard initial engine combustion, which can cause hard-starting.

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