Audi RSQ3 Intake Leak + MPG Drop, Urgent Help Guide

Had a similar issue on my 2016 Audi RS3 with the 2.5L engine. The symptoms match exactly what I experienced, poor fuel economy, hissing noise, and check engine light. The culprit was a cracked vacuum hose connecting to the intake manifold. The turbocharger was creating boost pressure, but the system was losing it through the leak. This caused the engine to run rich, leading to increased fuel consumption. First step should be checking all vacuum lines and intake boots for cracks or loose connections. In my case, the hose had degraded due to engine heat over time. This is a common issue on these engines, especially around the 120000km mark. The repair itself is not particularly complex, but proper diagnosis is crucial since similar symptoms can point to other issues with the intake manifold or turbocharger system. Would strongly recommend having a proper boost leak test performed. While not a catastrophic problem if addressed promptly, running with an intake leak can potentially cause long-term issues with fuel trim settings and turbocharger performance. For proper diagnosis, you will need specialized equipment to perform pressure testing of the entire intake system. This is not typically something that can be properly diagnosed without professional tools.

Thanks for sharing those details about the vacuum hose issue! The symptoms do sound identical to what I am experiencing. Good to know about the boost leak test being necessary for proper diagnosis. I am planning to have this checked soon based on your advice. Could you remember roughly how much the repair ended up costing you? Also curious if you have had any related issues with the intake manifold or air filter since getting it fixed? Want to be prepared if there might be some follow-up maintenance needed.

Thanks for asking about the follow-up. After getting the vacuum hose leak fixed at the shop, my intake system has been running perfectly. The repair cost was exactly 100 Euro for replacing the damaged intake hose, and the leak detection test confirmed there were no other issues. Since the repair, fuel economy returned to normal and that annoying hissing sound completely disappeared. The engine misfire symptoms also cleared up once the intake manifold was getting proper airflow again. I have had zero problems with the air system in the 30000km since the fix. Just make sure to have the throttle body cleaned during regular maintenance to prevent any future air intake issues. The key was catching and fixing the leak early before it could cause more extensive damage. One tip: worth having them check all the vacuum hoses while they are in there, as these tend to wear out around the same time. This preventive step can save you from future leaks.

Thanks everyone for the helpful information. I took my car to a new mechanic yesterday, and the diagnostic matched exactly what was discussed here. They found a cracked vacuum line near the intake manifold that was causing the boost leak. The repair was straightforward, total bill came to 180 Euro including parts, labor and the boost leak test. They also did a complete check of the air intake system while they had it open. Everything else looked good, including the throttle body and air filter. Car is running perfectly now, no more hissing sound and the check engine light is off. Fuel economy is already noticeably better after just one day of driving. Really glad I asked here first and knew what to expect at the shop. Will definitely keep an eye on those vacuum lines during future maintenance.