Question 44 UFIV01 - Chief Engineer - UFIV

The Correct Answer is D **Explanation for Option D (Correct Answer):** The core problem described is the engine becoming "air bound" (losing prime due to air intrusion) when a fuel system leak is suspected, with the fuel tank located beneath the engine (meaning the system uses a lift pump – the booster pump – to draw fuel upwards). In a system where the fuel tank is below the engine, the entire **suction line** (the line between the tank and the booster pump inlet) operates under a vacuum (pressure below atmospheric). If there is a leak (a crack, loose fitting, or pinhole) in this vacuum section, the lower internal pressure will cause atmospheric air to be sucked **into** the fuel line instead of fuel leaking **out**. This introduction of air directly into the pump inlet causes the engine to lose prime and become air bound. This is the most common location for air binding issues in systems with tank-below arrangements. **Explanation of Incorrect Options:** **A) 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 operate under extremely high pressure (thousands of PSI). A leak here would result in a highly visible and dangerous spray of fuel, not the subtle introduction of air into the low-pressure sections of the system. While a severe high-pressure leak could eventually starve the engine, it would manifest primarily as a massive fuel loss and performance issue, not the characteristic air-binding failure associated with a suction leak. **B) In the booster pump discharge line between the booster pump outlet and the fuel injection pump inlet.** * The booster pump (lift pump) pressurizes the fuel. Therefore, this line operates under positive pressure (typically 10 to 50 PSI). If a leak occurred here, diesel fuel would spray or drip out. Since the pressure in the line is higher than atmospheric pressure, air cannot enter the system at this location. **C) 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 carries excess fuel back to the tank, usually at low (but positive) pressure, or simply flowing by gravity. A leak here would result in fuel leaking out, not air entering the system. The fuel injection pump itself and the downstream components are kept full by the constant positive pressure supplied by the booster pump. A return line leak does not compromise the prime of the suction side.