ABYC Diesel Practice Exam

What is a potential cause of a rough engine idle according to Technician A?

1. Fuel shut-off solenoid malfunction

2. Air in the fuel system

3. Dirty fuel injectors

4. Faulty spark plugs

The correct answer is: Air in the fuel system

A rough engine idle can be caused by several factors, and one of the primary reasons is the presence of air in the fuel system. When air gets into the fuel lines, it disrupts the normal flow of fuel to the engine. This can lead to incomplete combustion, which causes the engine to run unevenly, resulting in a rough idle. Additionally, air in the fuel system can create bubbles that interfere with the proper functioning of the fuel injectors, further exacerbating the idle issue. Understanding the behavior of a diesel engine is crucial here, as it relies on the efficient delivery of fuel under pressure. Any introduction of air not only affects the engine’s performance but can also lead to stalling or hard starting since diesel engines are designed to operate with diesel fuel rather than a mix of fuel and air. While there are other contributing factors to a rough idle, such as a fuel shut-off solenoid malfunction, dirty fuel injectors, and faulty spark plugs, these options typically relate to different aspects of engine operation. Dirty fuel injectors can lead to poor atomization of the fuel, affecting combustion, but air in the fuel system is often a more direct and immediate cause of rough idling. Similarly, faulty spark plugs are more relevant to gasoline engines