What is the effect of a vacuum leak in the fuel system?

A vacuum leak in the fuel system introduces an excess of air into the engine without corresponding fuel to match it. This disrupts the intended air-fuel mixture, resulting in a lean mixture where there is too much air relative to fuel. A lean air-fuel mixture can lead to several issues, such as reduced engine performance, misfires, increased combustion temperatures, and potential engine knocking. Consequently, maintaining the proper balance of air and fuel is crucial for optimal engine operation, and the presence of a vacuum leak directly impacts this balance by causing the mixture to be improperly calibrated.