Swirl Flaps

They are controlled by an actuator and position sensor. Soot from EGR can make them stick. On some engines flaps have come loose and been drawn into the engine. Workshops often replace the whole intake manifold with flaps, or clean it and recalibrate the actuator.

At low load the flaps close partly to make the air swirl; at high load they open.

Sticking flaps cause codes and poor running. Removing them is not legal in many places and affects emissions.