Most vehicles today run on gasoline or diesel fuel. To make the vehicle move and work properly , an exact amount of air must be mixed with the proper amount of fuel burned in an engine.
The “perfect” fuel air mix is called a “stoichiometric fuel ratio.” This is 14.7:1, 14.7 parts air to1 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, give off a burning effect to the eyes 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”.
On modern computer controlled engines this will cause the computer to turn on an amber check engine light which will give a trouble code of “system too rich” or “system too lean”
Either condition will result in poor fuel mileage or possible engine damage.
A rich mixture will cause the vehicle to run hot. It will cause a clogging up off the catalyst resulting in the degradation of the honeycomb in it.
If your car is running too lean, it could result in jerking.
Possible causes of a rich mixture
– Dirty air filter
– Stuck open injectors
– Bad oxygen sensors
– Faulty ECU
– Bad Mass Airflow sensor
– Retrofitted cooling system (Thermostat removed, Fan running direct, double cell radiator)
– Bad Engine Coolant Temperature (ECT) Sensor
Possible causes of lean mixture
– Bad fuel pressure
– Vacuum leak
– Stuck open EGR valve
– pump regulator
– Insufficient fuel