When an injector is leaking, what symptom is most likely to occur?

When an injector is leaking, it allows fuel to escape outside of the normal flow path, leading to several significant symptoms that can adversely affect engine performance.

Hard starting can occur because the system may not build sufficient fuel pressure at initial crank, making it difficult for the engine to start, especially if the fuel leaks out entirely when the engine is off.

A rough idle is another common symptom resulting from a leaking injector since it introduces an imbalance in the air-fuel mixture. Too much fuel, due to the leak, can cause the engine to run rich, leading to uneven combustion and rough idling.

Decreased fuel economy is also likely. The leaking fuel increases the overall fuel consumption because the engine will require more fuel to compensate for what is wasted. This inefficient fuel usage contributes to lower miles per gallon.

Given that leaking injectors typically result in all these symptoms, the most comprehensive answer is that all the mentioned issues are interrelated consequences of a leaking fuel injector.