Return to the Econofix Home Page
Most all vehicles today run on gasoline or diesel fuel. In order to make the vehicle move, the fuel is mixed with air and burned in an engine. In order for this to work properly, an exact amount of air must be mixed with the proper amount of fuel. The "perfect" fuel air mix is called a "stoichiometric fuel ratio." This is 14.7 to one: 14.7 parts air to one part fuel. In reality most engines rarely achieve this ratio: they run a bit richer most of the time. an engine runs especially rich when accelerating, when cold, or when under a load.
If there is too much fuel and not enough air, the engine is said to be "running rich", or "has a rich mixture". It will have a gassy or rotten egg smell from the exhaust, and will make black smoke. If there is too much air and not enough fuel, the engine is said to be "running lean." or "has a lean mixture" It will have a foul-smelling exhaust that will burn your eyes, but no smoke. On modern computer controlled engines this will cause the computer to turn on a amber check engine light which will give a trouble code of "rich oxygen sensor reading" or "lean oxygen sensor reading"
Either condition will result in poor fuel mileage and possible engine damage. A lean motor will run hot, sometimes so hot the exhaust will glow red!!