An engine air filter blocks contaminants from entering the combustion chambers. Made of pleated paper, cotton, or synthetic fibers, it sits inside a sealed box in the intake tract. A running engine draws in thousands of cubic feet of air. The filter traps dirt, sand, and leaves, which would otherwise grind down cylinder walls, piston rings, and engine bearings like valve guides.
As the filter traps dirt, it slowly clogs. This restricts airflow, starving the engine of oxygen. The engine control computer will try to compensate, but you will experience poor acceleration, rough idling, and decreased fuel mileage. Inspecting and replacing this filter is one of the easiest and most cost-effective ways to maintain engine health.