The brief
The S350 had gone rough: an uneven idle, the odd misfire, the check engine light on, fuel economy down, the engine feeling flat, and an audible hiss from the engine bay. The owner brought it in. That list points at the intake manifold. On these V6s the manifold isn't just a casting, it has moving runner flaps inside that change the airflow path depending on engine speed. When the flaps or their linkage wear, or a section of the manifold cracks, you get a vacuum leak: unmetered air sneaking in past the throttle. The engine can't keep the air-fuel mix right with a leak it can't see, so the idle wanders, it misfires, and the light comes on. The hiss is the leak itself. A manifold that's worn or cracked needs replacing.
The diagnosis
Diagnostics confirmed it, fault codes for the intake manifold runner control and lean-mixture codes, and a leak test showed unmetered air getting in through the manifold. The runner flaps weren't moving cleanly. The injectors, the coil packs and the rest of the intake checked out. That's a manifold replacement. The runner flaps and their actuator are built into the manifold, you don't rebuild it on the car, so the call was a complete unit.
The work
The old intake manifold was removed, and a new genuine Mercedes-spec manifold fitted with fresh gaskets all round, the throttle body and connections cleaned up as it went back together. Then the fault codes were cleared and the adaptations reset so the engine could relearn its fuelling against a manifold that wasn't leaking. A road test confirmed a steady idle, no misfires, the light staying off, and the response back.
The outcome
Smooth idle, clean throttle response, no misfires, no warning light, and no hiss from the bay. The S350 went home running properly again. An intake manifold with worn flaps or a crack only leaks worse over time and keeps the engine fighting its own fuelling, so changing it and letting the engine relearn put the running right.