Hard Starting, Rough Idle, Black Exhaust Smoke, Poor fuel mileage may indicate a leaking fuel injector or more commonly a leaking injector O-ring. Our technicians tell us that with engine idling look into the throttle body with a flashlight, If fuel is seen dripping from around the injector, the O-ring is likely leaking. If fuel is dripping from the injector nozzle, replace the injector.