If your car engine is running rich, there's too much fuel in the engine cylinders and less oxygen. When this happens, fuel will not burn properly, and the engine will produce less power than it should if the air-fuel ratio is stable. A faulty MAF sensor is a common cause of the rich condition in engines. If you have a car running rich, it means that the air-fuel ratio of your engine is not correct. The ratio should be 14 parts of air to 1 part of fuel. This ratio is called the stoichiometric ratio, and it is the ideal ratio for an engine to burn fuel efficiently and produce maximum power. A car running rich will usually have black smoke coming from the exhaust. Smoking is caused by unburned gas entering the combustion process; it leaves through the tailpipe. It also causes increased fuel consumption.