Question 19 UFIV02 - Assistant Engineer - UFIV

The Correct Answer is D **Explanation for why Option D is correct:** The problem states that the diesel engine fuel system is prone to becoming "air bound" (losing prime) and that a piping leak is suspected. The fuel tank is located *beneath* the engine, meaning the booster pump (or lift pump) must actively draw fuel upwards against gravity (negative head) from the tank to the engine. If a leak exists in the **booster pump suction line (D)**, the pump will not only pull fuel but also draw air into the system whenever the engine is running or when it attempts to start. Because the line is under suction (vacuum), the leak allows air entry. When the engine is shut down, the fuel in the suction line will tend to drain back into the tank due to gravity and the vacuum being relieved, causing the booster pump to lose its prime and introducing a large air pocket. This makes the system "air bound" and difficult or impossible to restart without manual priming. **Explanation for why the other options are incorrect:** * **A) In the booster pump discharge line between the booster pump outlet and the fuel injection pump inlet:** This section of the piping is under positive pressure (pump discharge). If a leak occurs here, fuel would spray outward, but air would generally not be drawn in. While a severe leak could potentially drop the pressure enough to cause problems or introduce vapor, the primary symptom of a leak in a positive pressure line is fuel loss, not air ingestion leading to being air bound, especially after shutdown. * **B) In the fuel injection pump return line between the fuel injection pump return outlet and the fuel oil day tank (or booster pump suction as appropriate):** The return line is also under positive pressure (albeit low pressure) as it routes excess fuel back to the tank. A leak here would result in fuel spillage, not air binding. It would not cause the main suction line or the booster pump to lose prime. * **C) In the fuel injection pump high pressure fuel lines between the fuel injection pump high pressure outlets to the fuel injector nozzle inlets:** These lines are under extremely high pressure (thousands of psi). A leak here would result in immediate, catastrophic fuel spray and dramatic loss of engine power (a misfire), but the high pressure prevents air from being drawn into the system. This type of leak would not typically lead to the general "air bound" condition described, which is usually a low-pressure priming issue.