When a car has a vacuum leak that means that air is leaking into the engine in places that should be sealed. The leak reduces the engine's overall efficiency and impacts the function of a variety of systems that rely on a sealed vacuum system. If yÒ€¦ consult the vacuum diagram of your engine. in order to understand where your engine's vacuum hoses are and what areas might be leaking, you can look at a diagram that details how the vacuum system works.