Why Does My Exhaust Smell Like Gas & My Car Have a Rough Idle?

by Justin CuplerUpdated July 10, 2023
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Exhaust Smells like Gas?

Experiencing a gas smell in your vehicle or a check engine light? The issue could be as simple as unscrewed gas cap and subsequent gas leak.

However, the internal combustion engine could be the problem. Engines requires a precise mixture of fuel and air intake to run correctly. Too much of either can result in severe running issues. When your engine idles rough and the exhaust fumes have the smell of raw, unburned gasoline, it is suffering from a rich condition or or incomplete combustion. There are four main component failures that can result in this condition.

Fouled or Worn Out Spark Plugs

Over time, or due to mechanical problems, the spark plugs in your vehicle wear out or foul. The actual act of a spark plug wearing out is when the electrode wear out, widening the spark plug's gap. A widened gap causes a colder spark, which can create incomplete combustion, as it cannot burn all of the fuel in the combustion chamber. This allows raw, unburned fuel to escape the exhaust and tailpipe.

Fouled plugs have a foreign substance -- ash, soot, tar, oil or fuel -- coating the electrodes, causing excessive resistance and making the spark colder. This results in a rough idle and a fuel smell in the exhaust system.

Failed Ignition Coil(s) or Distributor

The ignition coil(s) and distributor supply the electrical current to the spark plugs, so that a spark can occur. If the coil(s) or distributor fails, the spark may be too cold to ignite all of the fuel in the combustion chamber. The symptom is a rough idle and a exhaust smell in from the exhaust.

Leaking Fuel Injector or Carburetor

The fuel injectors or carburetor on your vehicle regulates fuel flow into the combustion chamber. If an injector or carburetor begins leaking fuel into the combustion chamber, it creates a rich running condition. This results in a rough idle and unburned gas making it into the fuel system, creating a gasoline smell from the exhaust pipe.

Faulty Computer

From the mid-1980s to present day, computers have become more prevalent in automobiles. The vehicle's computer regulates everything from the power windows to fuel and air mixture. If the vehicle's computer fails, it may not read the air/fuel ratio correctly for fuel pressure. This may result in the computer thinking that the engine is running lean -- not enough gasoline -- and increase the flow of fuel into the combustion chamber. This will result in a rough idle and a smell of gas in the exhaust.

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