The swirl flaps sit in the intake of both cylinder banks, and a central electric actuator moves them through a linkage. Soot builds up on the linkage and on the flaps, the flap starts to bind, and in the end the plastic breaks. This is the most frequent single failure on this engine, and the broken pieces can get into the engine, which can lead to engine damage. The symptoms are the engine warning light, rough running and a lack of power, with the code P2004 or P2005 in the fault memory. The condition of the flaps can be seen with an endoscope through the intake, without dismantling. It is worth looking above 150,000 km, especially on the W164 ML320 and ML350 CDI. An aluminium repair kit is available for it, which lasts longer than the factory part.