Vacuum Leak
An unintended opening in the engine's intake system downstream of the throttle body, allowing unmetered air to enter the cylinders.
A vacuum leak is a common engine fault where air enters the intake manifold downstream of the throttle body and mass airflow (MAF) sensor. Because this air does not pass through the throttle body or MAF sensor, it is not measured (unmetered) by the engine computer. This upsets the calculated air-fuel ratio, resulting in an excess of oxygen (a lean running condition).
Symptoms of a vacuum leak include a rough, erratic idle, engine hesitation or stumbling during acceleration, stalling at stops, and sometimes a whistling or hissing sound from the engine bay. The ECU will attempt to correct the lean mixture by adding fuel, raising the long-term fuel trim (LTFT) values, and will eventually trigger check engine light codes such as P0171 or P0174.
Common locations for vacuum leaks include cracked rubber vacuum hoses, a torn air intake boot, a leaking intake manifold gasket, a stuck-open PCV valve, or leaking brake booster diaphragms. Technicians diagnose vacuum leaks by spraying a mist of water, throttle cleaner, or using a professional smoke machine to inject smoke into the intake, looking for where it escapes.