What effect does a vacuum leak have on the air-fuel mixture?

What effect does a vacuum leak have on the air-fuel mixture?

• A. It leans the mixture
• B. It richens the mixture
• C. It has no effect
• D. It saturates the air

Answer: (A)

A vacuum leak introduces extra, unmetered air into the intake. The engine management system meters fuel based on the air it senses, so when additional air sneaks in, the actual air goes up while the calculated fuel stays the same or doesn’t increase enough. The result is a lean air-fuel mixture—more air relative to fuel. This lean condition is most noticeable at idle or light throttle, where the engine relies on a precise balance to run smoothly. You might feel rough idle, hesitation, or misfires as the mixture runs lean.

A vacuum leak introduces extra, unmetered air into the intake. The engine management system meters fuel based on the air it senses, so when additional air sneaks in, the actual air goes up while the calculated fuel stays the same or doesn’t increase enough. The result is a lean air-fuel mixture—more air relative to fuel. This lean condition is most noticeable at idle or light throttle, where the engine relies on a precise balance to run smoothly. You might feel rough idle, hesitation, or misfires as the mixture runs lean.