S4 TDI Rough Running + Hissing Points to Intake Flaps

Had the same intake system problems on my A4 2.0 TDI. The symptoms matched exactly, power loss, hissing, rough running, and increased fuel consumption. The intake manifold control unit was failing, causing incorrect airflow control and improper engine performance. The repair required replacing the complete intake manifold assembly because the flap actuator had seized, leading to inconsistent air distribution. This is a known weak point in these engines. The mechanic found metal debris inside the intake system, which indicated the flaps had partially deteriorated. This is a serious issue that needs prompt attention. If left unfixed, it can lead to further engine damage and even more expensive repairs. The complete job involved: 1. Full diagnostic scan 2. Removal of intake manifold 3. Inspection of all flap mechanisms 4. Installation of new manifold assembly 5. Control unit reprogramming 6. Systems check After repair, the engine regained proper power delivery and smooth operation. The air intake system now functions correctly, and fuel consumption returned to normal levels. Best to have a qualified technician with specific experience on VAG engines handle this repair.

Thanks for the detailed write-up. Your situation sounds identical to what I faced with my S4. After dealing with engine misfire symptoms and similar reduced power issues, replacing the intake manifold assembly fixed everything. Did you track the fuel consumption after the repair? Would be interested to know if yours went back to normal like mine did. Also, do you remember the repair cost? Been seeing others mention additional issues popping up later due to the metal debris, but mine has been running great for the past few months.

Since getting the airflow control system fixed, my fuel consumption dropped back to factory specs. The repair bill came to 465 Euro for replacing the switch intake manifold control unit. Money well spent considering how the engine misfire and reduced power issues completely disappeared. The mechanic did a thorough cleaning of the intake system to remove any metal particles, which was crucial to prevent future problems. No additional issues have surfaced in the months following the repair. The car runs smooth with proper air intake performance, just like when it was new. I keep monitoring the engine performance closely, but the flap control system has been working flawlessly since the fix. Unlike some others, I caught the problem early enough to prevent any secondary damage to other engine components.

I took it to my regular mechanic last week after seeing your previous replies. You were right, it was exactly the intake manifold issue. The repair cost me 520 Euro, slightly more than yours, but included a complete system flush to remove any debris. The engine performance difference is like night and day. The air intake system works perfectly now, no more hissing or jerking. My fuel consumption dropped by about 15% after the airflow control fix, which actually makes the repair cost worth it in the long run. The mechanic showed me the old flap control unit, and it was completely shot. Small pieces of metal were visible in the manifold, just like you mentioned. They cleaned everything thoroughly and installed a newer version of the intake manifold that supposedly has better durability. Three weeks in, and the engine runs beautifully with proper power delivery. No warning lights or weird noises since the repair. Thanks for helping point me in the right direction, saved me from going through multiple unsuccessful repair attempts.