The internal combustion engine requires a precise mixture of fuel and air to run correctly. Too much of either can result in severe running issues. When your engine idles rough and the exhaust has 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.
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.
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.
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 gasoline smell in from the exhaust.
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 exhaust, creating a gasoline smell in the exhaust.
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 mixture correctly. 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.
