A failing Cummins injector cup can cause several noticeable symptoms, including:
Coolant in the fuel system (milky fuel or oil)
Engine misfires or rough idling due to improper fuel sealing
White smoke from exhaust (indicating coolant burning)
Loss of coolant without visible leaks
Since injector cups are sealed inside the cylinder head, diagnosing them requires pressure testing or a dye test. If ignored, a bad injector cup can lead to injector damage or even hydro-lock, which can severely harm the engine.
Early detection and replacement can save you from expensive repairs down the road.