S3 Turbo Leak Warning Signs: Hiss, Fuel Waste & CEL

Had almost the exact same issue with my 2018 S4. With some experience working on cars, I can say vacuum hose problems are pretty common on these engines and can cause the symptoms you describe. The engine misfire and increased fuel consumption are typical signs of an air leak in the intake system. My case turned out to be a cracked vacuum hose near the throttle body, which was letting in unmetered air. The workshop used smoke testing for leak detection, which quickly revealed the problem. The repair itself was straightforward, replacing the faulty hose and checking the surrounding connections. Total cost was 100 Euro including parts and labor. Since the fix, fuel economy returned to normal and the hissing disappeared completely. Before making any conclusions, could you share: Any diagnostic codes from the check engine light? Is the hissing sound constant or only under specific conditions? Have you checked your air filter condition? When was your last throttle body cleaning? These details would help determine if your issue matches what I experienced or if it might be something else in the intake system.

Last service was at 44464 KM and the workshop did a thorough throttle body cleaning. The check engine light showed code P0171 (system too lean). The hissing is most noticeable under acceleration and seems to come from the passenger side. The air filter was replaced during service but I will check it again. Smoke testing sounds like a good approach for proper leak detection. Will inspect all vacuum hoses myself first, focusing on the area you mentioned near the throttle body. Thanks for the specific repair cost reference, helps with budgeting if I need professional help.

Thanks for the follow-up info. That P0171 code is exactly what I had, and with the hissing location you described, it strengthens my suspicion about the vacuum lines. The timing of your symptoms relative to the service is interesting, sometimes vacuum hoses can get damaged during maintenance work. When my turbocharger was inspected during service, a hose got slightly pinched, which later developed into a crack. I did a basic check before going to the shop by spraying soapy water around the intake manifold area while the engine was running, bubbles formed near the problematic hose. While not as effective as professional leak detection equipment, it helped identify the general area. In my case, the throttle body was actually fine despite the recent cleaning. The real culprit was that cracked vacuum line causing unmetered air to enter the system, throwing off the fuel economy and triggering the lean condition code. Quick question, does the hissing get louder when you rev the engine while stationary? This detail helped pinpoint my leak location.

The P0171 code and location description match up with my situation. The hissing does get louder when revving, most noticeable around 2000-3000 RPM while stationary. I did the soapy water test you suggested and found bubbling near one of the vacuum hoses connected to the intake manifold. Pretty sure this confirms what you experienced with the cracked line. Going to book it in for proper leak detection testing to be certain. The 100 Euro repair cost seems reasonable if it turns out to be just a vacuum hose replacement. Smart suggestion about checking for damage that might have occurred during the recent service, will mention this to the workshop. At least I can rule out major issues with the throttle body and air filter given the recent maintenance. Really helpful to hear about your identical experience with the engine misfire symptoms and how a simple hose replacement solved everything.