What troubleshooting step could indicate a fault in the fuel system?

Fluctuating idle speed can be a key indicator of a problem within the fuel system. This symptom often points to issues such as inadequate fuel delivery, improper fuel mixture, or malfunctioning fuel components like injectors or the fuel pump. When the fuel system cannot maintain a consistent supply of fuel, the engine may struggle to run smoothly, leading to variations in idle speed. These fluctuations can stem from various reasons, including air leaks, faulty sensors (like the mass airflow or throttle position sensor), or a dirty fuel filter that restricts fuel flow.

Addressing fluctuating idle speed usually requires a thorough examination of the fuel system to ensure proper functionality, making it a critical troubleshooting step when diagnosing engine performance issues.