What effect does a blocked or restricted air filter have on engine performance?

A blocked or restricted air filter limits the airflow into the engine, which is crucial for the combustion process. When the air supply is reduced, the engine gets less oxygen needed for proper combustion. This imbalance often leads to a condition known as running "rich," where there is an excess of fuel compared to the amount of air available for mixing. Consequently, the engine cannot burn all of the fuel efficiently, resulting in incomplete combustion. This can lead to various performance issues, such as reduced power output, increased exhaust emissions, and potential damage over time due to fuel buildup.

In contrast, the other options do not align with the effects of a restricted air filter. For example, if the air filter were blocked, it would not cause the engine to run lean; in fact, the opposite occurs because less air leads to a higher fuel-to-air ratio. Additionally, while a restricted filter can contribute to overheating due to inefficient combustion and excess fuel, it does not directly cause the engine to overheat as the primary issue. Lastly, a blocked air filter will not improve fuel efficiency; rather, it typically leads to decreased efficiency owing to poor combustion and increased emissions.