The most obvious sign that a car engine has a vacuum leak is when a person hears a whistle or hiss coming from the engine. Some other signs of a vacuum leak include an engine speed that is idling too fast, a rough idle, stalling and acceleration misfire.
The less obvious signs of a leak in a car engine vacuum are commonly confused for ignition or fuel problems that are hard to diagnose. The presence of a vacuum leak is more noticeable when the car is idling, and less noticeable when the car at part or full throttle. This is due to the amount of airflow to the engine increasing during the throttle.