What is the most common cause of black smoke emitted from an engine?

Black smoke from an engine is primarily a result of excess fuel being burned in the combustion process. When there is too much fuel compared to the amount of air available for combustion, it leads to incomplete combustion. This incomplete combustion generates soot particles, which appear as black smoke exiting the exhaust.

Excessive fuel can occur due to several reasons, such as a faulty fuel injector, a malfunctioning fuel pressure regulator, or a clogged air intake system that restricts airflow. This situation causes the combustion chamber to receive an imbalance of fuel and air, ultimately resulting in black smoke.

Other factors, like burning oil or a clogged air filter, can contribute to different kinds of emissions. Leaking coolant typically results in white smoke, not black. Thus, the primary reason for the black smoke observed is indeed the presence of excessive fuel in the combustion chamber.