Swirl flaps sit in the intake ports of the six cylinder M57 diesel: at part load they spin the incoming air, saving fuel and emissions. According to the German technical press these flaps were made of metal on the engine versions before the technical revisions (TU and TU2), and sustained use can break the mechanism.
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That is how it becomes engine damage: the detached flap travels down the intake port into the combustion chamber, and destroys the piston and the cylinder head there. That is not a repair but an engine rebuild.
Not every M57 has swirl flaps. Fitment depends on the model year, the gearbox and the emissions standard. Since the first revision (TU) the mechanism is plastic and more reliable, so this applies to the early engines from before that revision.
What to check
- Ask whether the swirl flaps have already been removed (many M57 owners do it preventively): if so, ask for the job sheet
- Ask for an endoscope inspection, or at least a visual check of the flaps from the intake side
- Listen to the engine under load: irregular running, loss of power and a warning light are suspect