Swirl flaps are some of the most abnormal components in the construction of a BMW diesel engine. They are normally rested in a closed position. They open up under a few circumstances. For example, they stay open if the engine speed is higher than 2,250RPM. Since they open and close so often, they risk getting stuck in one position or another. This is when problems arise. Unfortunately, the initial symptoms are relatively mild, so no one will notice them. They become obvious when you already need a sophisticated reparation. It might be wiser to just check these components on a regular basis.