The Fan Idler Pulley, also known as the idler pulley or tensioner pulley, is a component of a truck's engine system that helps regulate the tension and movement of the serpentine belt. This part is responsible for maintaining proper belt tension, preventing slippage and excessive wear on both the belt and other engine components. The Fan Idler Pulley rotates on a bearing mounted to a bracket attached to the engine block. Its smooth surface allows for consistent contact with the serpentine belt, ensuring that it remains in place and operates smoothly.