What occurs when unmetered air enters the engine's intake system?

When unmetered air enters the engine's intake system, it creates a situation known as a vacuum leak. This occurs when the air entering the system is not accounted for by the engine's mass airflow sensor or other metering devices. Since these sensors help to determine the amount of fuel that needs to be injected for proper combustion, unmetered air can cause the air-fuel mixture to become imbalanced.

As a result, the engine may run lean (too much air compared to fuel), leading to poor performance, misfires, increased emissions, and potential engine damage if left unaddressed. In essence, a vacuum leak disrupts the precise air-fuel ratio that is essential for optimal engine operation, making it critical to identify and rectify any breaches or leaks in the intake system.

The other options pertain to different types of leaks (coolant, oil, and fuel) that have distinct symptoms and causes unrelated to the intake air flow issues resulting from unmetered air.