What is the effect of a faulty spark plug on engine performance?

A faulty spark plug can significantly impact engine performance, and one of the most noticeable effects is a rough idle. This occurs because a malfunctioning spark plug fails to ignite the fuel-air mixture effectively in the combustion chamber. When combustion is irregular or inconsistent due to the poor spark, it leads to engine vibrations and uneven power output, which manifests as a rough or unstable idle condition. The engine may also misfire, which further exacerbates the roughness during idling.

While the other options suggest positive outcomes such as increased fuel efficiency, improved acceleration, or reduced engine noise, these are generally not associated with a faulty spark plug. In fact, rather than improving performance, a faulty spark plug can lead to reduced fuel efficiency and impaired acceleration due to incomplete combustion and the resultant loss of power. Therefore, recognizing a rough idle as the primary consequence of a faulty spark plug is essential for diagnosing and maintaining optimal engine performance.