What Are the Most Common Signs of a Failing Injector? A failing Common Rail injector can cause significant performance issues. Watch for these key symptoms: Hard Starting or Misfiring: Difficulty starting the engine, especially when cold, or a noticeable misfire/rough idle as the cylinder with the bad injector fails to fire properly. Poor Fuel Economy and Excessive Smoke: An injector that is stuck open or leaking will dump too much fuel into the cylinder, leading to unburnt fuel (black smoke) and a sudden drop in miles per gallon. Loss of Power and Engine Knock: An injector that is clogged or not delivering enough fuel will cause that cylinder to underperform, leading to a overall loss of engine power. You might also hear a loud knocking sound from unbalanced combustion. Fuel Smell and Contaminated Oil: A leaking injector can allow fuel to drip into the crankcase, diluting your engine oil. You might smell raw fuel, and your oil level may appear to rise on the dipstick—a very serious condition that requires immediate attention. https://www.cq-longshine.com/cummins-common-rail-injector-and-pump/cummins-common-rail-fuel-injector-0445120361