Question 49 MODE01 - Chief MODU Engineer

The Correct Answer is C **Why Option C ("Air in the fuel system") is correct:** Air in the fuel system is the single greatest threat and main source of malfunction for modern high-precision fuel pumps and injection systems, particularly in diesel engines but also problematic in gasoline systems. Air (or gas cavitation) causes several critical issues: 1. **Loss of Prime and Pressure:** Pumps, especially positive displacement pumps, are designed to move incompressible fluids (liquid fuel). If air is introduced, the pump struggles to build and maintain the extremely high pressures required for injection, leading to stalling, hard starting, or complete failure to run. 2. **Lack of Lubrication and Cooling:** Fuel is the primary lubricant and coolant for the precision internal components of the fuel pump (e.g., plungers, rollers, cam lobes) and injectors. Air provides no lubrication. When air runs through these components, it results in metal-to-metal contact, causing rapid overheating, scoring, wear, and catastrophic internal failure of the pump within minutes. **Why the other options are incorrect:** * **A) Coated fuel lines:** While coated or restricted fuel lines (due to sludge, varnish, or corrosion) will cause fuel starvation and poor performance, they typically lead to low pressure and fuel delivery issues, not the catastrophic, rapid pump failure associated with the loss of lubrication caused by air. They are a secondary source of poor performance, not the main source of *malfunction* (failure). * **B) Excessive vibration:** Excessive vibration can accelerate wear on mounting components and fittings, potentially causing leaks or cracks over time. However, the pump and injection system components are designed to tolerate the engine's normal vibration. It is a source of component wear, but not the primary cause of immediate internal failure or system malfunction. * **D) Improper lubrication:** This is essentially a misleading option. Fuel pumps and injectors are *lubricated by the fuel itself*. They do not rely on a separate oil system (unlike the engine). Therefore, the only way lubrication becomes "improper" is if the lubricating medium (fuel) is replaced by air (Option C) or contaminated by a harmful agent (like water), making C the more direct and accurate answer regarding system malfunction.