The location of the pool pump diffuser is shown in this cut-away of a pump. The pool pump diffuser surrounds the pump impeller. It's purpose is to control the flow of water to and from the impeller. For a more complete discussion of the function of the diffuser click How A Pool Pump Works. Sometimes hard debris will get into the pump and will crack the diffuser. In other cases the threads that hold the impeller to the motor shaft become worn causing the impeller to wobble and rub against the diffuser. This guide explains how to replace a bad pool pump diffuser in a Hayward Pump, however, the pool pump's parts and it's operation are common to most pool pumps.