The brief
The A4 had a few things going on at once. A faint hiss under acceleration. A rough idle. Fuel economy that had dropped on the owner's regular routes. And a check engine light.
Several symptoms, all on the intake side. A hiss under acceleration usually means air getting in or out where it should not. A rough idle and worse economy follow when the engine cannot accurately control its airflow. And the check engine light meant the engine's own monitoring had noticed something was off.
All of it pointed at the intake manifold.
The diagnosis
The codes pointed at the intake manifold's swirl flap actuator: the runner position sensor reporting out of range. On this engine the intake manifold has flaps inside it that change the air's path at different revs, and an actuator that moves them.
Looking through the throttle body, the picture got clearer and worse. There were broken pieces of the swirl flap loose inside the intake tract. That changes the job completely. Replacing just the actuator would leave the broken plastic rattling around in there, ready to get sucked into the engine. The whole manifold had to come off, the broken pieces recovered, and a new manifold fitted.
The work
Removed the intake manifold. Recovered the broken swirl flap pieces from inside the tract, accounting for them so nothing was left behind to find its way into a cylinder.
Fitted a new VAG-spec replacement manifold, complete with all its gaskets and a fresh actuator. Reseated the harness, then cleared the stored fault codes on the scanner.
The key part of this job is the recovery. A new manifold is straightforward; making sure every fragment of the old broken flap is out of the engine is the bit that takes care.
The outcome
No hiss under acceleration. Idle smoothed out. Throttle response back. Fuel economy returned to normal across the next tank. Check engine light cleared and stayed off.
The A4 went home running properly, with a manifold that does its job and no loose debris inside the intake. For the owner, the practical wins are a smooth engine, a fuel bill back where it should be, and, more importantly, a problem caught before a broken plastic fragment could get drawn into a cylinder and turn a manifold job into an engine job.
