The brief
Mr Li Wei's A4 had been vibrating while driving, the kind of shake that comes with a rough idle and an uneven run, and at 120,833 km it had earned a few wear-and-tear jobs. A check traced it to the intake manifold. On this engine the intake manifold isn't just a casting, it has moving runner flaps inside that change the airflow path depending on engine speed, driven by an actuator. When the flaps or the actuator wear, or a section of the manifold cracks, the airflow goes off and you can get unmetered air sneaking in, so the engine can't keep the air-fuel mix right, the idle wanders, it runs rough, and the management trips the light. The flaps and the actuator are built into the manifold, you don't rebuild it on the car, so a failed manifold means a complete unit.
The diagnosis
Diagnostics confirmed it, fault codes for the intake manifold runner control and a check showing the runner flaps weren't moving cleanly, with a touch of unmetered air getting in. The injectors, the coil packs and the rest of the intake checked out, it was the manifold. That's a manifold replacement, a complete unit with fresh gaskets, then the codes cleared and the engine let to relearn its fuelling.
The work
The old intake manifold was removed, and a new genuine Audi-spec manifold fitted with fresh gaskets all round, the throttle body and the connections cleaned up as it went back together. The fault codes were cleared and the engine's fuelling adaptations reset so it could relearn against a manifold that wasn't leaking and flaps that moved cleanly. A road test confirmed a steady idle, no vibration, the running smooth, and the light staying off.
The outcome
Smooth idle, clean throttle response, no vibration while driving, and no warning light. The A4 went home running properly again. An intake manifold with worn flaps or a crack keeps the engine fighting its own fuelling and the running rough, so changing it and letting the engine relearn put it right, the kind of wear that catches up with a car at this mileage.
