A leaking diesel engine fuel injector will likely cause ______.

A leaking diesel engine fuel injector creates an issue where fuel is not delivered in the measured amounts necessary for optimal combustion. Instead of the fuel being atomized properly and injected in a controlled manner, the injector allows an excess of fuel to escape, which can lead to an imbalance in the air-fuel mixture in the combustion chamber.

As a result, not all of the fuel is combusted completely during the combustion cycle. This incomplete combustion can produce a variety of negative outcomes, including increased emissions, loss of engine efficiency, and the potential for engine damage over time due to the buildup of unburned fuel. Therefore, a leaking fuel injector is directly linked to the condition of incomplete combustion, making this the correct answer.