What is a potential cause of a vehicle running rough at idle?

A vehicle running rough at idle can often be attributed to a vacuum leak. This occurs when there is an unintended gap in the intake system where air can enter the engine without passing through the throttle body, causing an imbalance in the air-fuel mixture. The engine control unit (ECU) is calibrated for a specific ratio of air to fuel, and a vacuum leak introduces excess air, leading to a lean mixture.

When this imbalance is present, the combustion process becomes erratic, resulting in rough idling. The engine may exhibit symptoms like stumbling, stalling, or increased RPM fluctuations. Mechanics often look for signs of a vacuum leak such as hissing noises, and they may perform tests to check for leaks in hoses and connections.

While other factors listed can also affect engine performance, they do not typically present specifically with rough idling in the same way as a vacuum leak does. For instance, a dirty air filter influences airflow but usually impacts performance at higher loads rather than idle. A faulty fuel pump primarily affects fuel delivery under load, leading to issues when accelerating or driving, rather than at idle. An exhaust leak may also create noise and could cause some roughness in engine operation, but it generally does not impact the air-fuel mixture