How does a vacuum leak affect engine performance?

A vacuum leak allows unmetered air to enter the intake system, which disrupts the air-fuel mixture that the engine control unit (ECU) intends to maintain for optimal combustion. When unmeasured air is introduced, it creates an overly lean mixture, meaning there is more air relative to fuel in the combustion chamber than required. This lean condition leads to a series of performance issues, such as a rough idle and potential stalling as the engine struggles to maintain stable operation due to insufficient fuel for the combustion process.

Additionally, an overly lean mixture can cause the engine to run hotter, which may lead to pre-ignition or knocking, further compounding performance issues. Furthermore, in prolonged cases, it may cause damage to engine components due to excessive heat or incomplete combustion.

The other options do not accurately reflect the consequences of a vacuum leak: excessive fuel consumption is typically a result of a rich mixture rather than a lean one, improving air-fuel mixture quality contradicts what happens with a vacuum leak, and stating that there is no effect ignores the obvious performance issues that arise from a lean condition.