Final answer:
A bad vacuum in an engine typically means a vacuum leak that introduces extra air into the mixture, causing it to become lean by increasing the air-to-fuel ratio.
Step-by-step explanation:
When discussing how a bad vacuum affects the mixture in an engine, it's important to understand that the vacuum in an engine affects the air-to-fuel ratio. A bad vacuum, which generally means a vacuum leak, introduces more air into the combustion chamber than intended. This excess air causes the mixture to lean out, meaning there's not enough fuel for the amount of air (a high air-to-fuel ratio), which can lead to poor performance, higher engine temperatures, and potential engine damage.
So, the correct answer to the student's question is 1) It causes the mixture to become lean. Having too much air and not enough fuel can cause the engine to run less efficiently and increase the risk of engine knock or pre-detonation, which can severely damage engine components.