P0171 OBD2 Code - 2002 Audi A3 1.8 Turbo: Vacuum Leak Diagnosis and Recirculation Valve Component Identification

Hi everyone, I recently purchased a 2002 Audi A3 1.8 Turbo and after driving it for a few days, the Engine Light (EML) came on. Using my friend's OBD2 scanner, I retrieved fault code P0171. After some research, I believe this is likely caused by a vacuum leak—especially since I can hear a noticeable sucking noise near the recirculation valve area. However, I'm struggling to locate the exact source of the leak. The car runs fine overall, though there's a faint petrol smell coming from the rear and fuel efficiency has dropped to around 36 MPG during highway driving at 75 mph (maintaining 3000 RPM). Additionally, I'd like to know what the black plastic component located right next to the recirculation valve is called—any guidance on this would be greatly appreciated. Any experienced owners or technicians with photos or troubleshooting tips for P0171 in a 2002 Audi A3 1.8 Turbo? Thanks in advance, Nubie.