Question 59 3AE01 - Third Assistant Engineer

The Correct Answer is B **Explanation for Option B (Correct Answer):** Air removal (bleeding) is essential in a diesel fuel system because air pockets can stop the flow of fuel to the high-pressure injection pump or the injectors, preventing the engine from starting or running smoothly. The process typically involves using a hand primer pump (if available on the engine) or running the lift pump to push fuel through the system while simultaneously opening a high point in the system where air tends to accumulate. Loosening the compression nuts (or bleed screws, if fitted) at the injectors, or sometimes at the fuel filter housing or injection pump outlet, allows the air mixed with the fuel to escape until only a steady stream of pure fuel emerges. Once the steady stream is observed, the nut is tightened, sealing the system and completing the bleed process. **Explanation for Incorrect Options:** **A) blowing down the air tanks:** This refers to draining moisture and contaminants from the compressed air storage tanks (used for starting the engine, controls, or air brakes). This action has no relation to the diesel fuel system or bleeding air from fuel lines. **C) changing fuel filters:** While changing fuel filters often introduces air into the fuel system, requiring subsequent bleeding, the *act* of changing the filters itself is the cause of the air ingress, not the method used to bleed the air out. The bleeding process occurs *after* the filters are changed. **D) pumping down the day tanks:** Day tanks hold the ready supply of fuel for the engine. "Pumping down" usually means draining the fuel from the tank, typically to remove sludge and water from the bottom. This action addresses fuel quality issues in the tank but does not remove trapped air from the pressurized fuel lines and components leading to the injectors.