Black smoke coming from a vehicle's exhaust system is a sure sign that the engine is consuming too much fuel. The actual cause of the increased fuel consumption varies, but it is often fairly easy to diagnose.
Specific causes are usually found in a vehicle's fuel system. Black smoke from the exhaust may occur when the air filter becomes clogged, resulting in an improper fuel to air mix. Other intake components may also cause fuel to feed too quickly into the engine. Having these components checked by a professional is the best way for car owners to ensure that a vehicle remains functional and that no unnecessary money is wasted on poor fuel economy.