Which injector part fails most often on older Cummins engines? On older PT and CELECT injectors, the plunger and barrel wear out most frequently. Over time, the tight clearance between them (measured in ten-thousandths of an inch) opens up due to fuel contamination or normal wear. This causes fuel leakage past the plunger, resulting in hard starting, misfires, and white smoke. The check ball and seat is the second most common failure point, leading to fuel dribble and injector knock. Replacing these parts as matched sets restores proper performance. https://www.cq-longshine.com/cummins-injector-parts