VW Fox Diesel Hissing + Poor MPG Points to Vacuum Leak

My VW Polo 1.6 TDI from 2014 had nearly identical symptoms. The hissing noise during acceleration was caused by a cracked air intake hose between the turbocharger and intercooler. This created a boost leak affecting the turbocharger performance and throwing off air-fuel ratios. The air intake system diagnosis revealed several issues: Deteriorated rubber intake hose sections, Loose clamps near the throttle body, Cracked vacuum lines, Dirty air filter restricting proper airflow A complete inspection of the intake system was necessary since multiple components were affected. The turbocharger was fortunately undamaged, but continued driving with the leak could have led to more serious problems. The repair involved: Replacing the main intake hose, Installing new vacuum lines, Changing the air filter, Properly securing all connections, Clearing fault codes After repairs, fuel economy returned to normal and the engine regained full power. These symptoms should not be ignored as they can lead to reduced turbocharger life and engine performance issues if left unaddressed. A proper diagnostic scan and pressure test of the intake system would be the recommended first step to pinpoint the exact location of the leak.

Thanks for sharing your experience. That sounds exactly like what I am dealing with now! The poor fuel economy and turbocharger issues are very similar. Did you track how much the total repair cost ended up being? Also curious if you had any related problems pop up since getting it fixed? My Fox is showing the same symptoms and I want to budget properly for the repairs.

Just following up, got the repair done last week and it was exactly what I suspected. The mechanic found a damaged vacuum hose and a small tear in the air intake system, causing that annoying hissing sound. The total repair was surprisingly affordable at 100€, which covered replacing the intake hose and fixing all the loose connections. The fuel economy has completely returned to normal now that the air intake system leak is fixed. The engine runs much smoother with no more hissing during acceleration. Your Fox should respond similarly well to the repair since it seems to be showing identical symptoms to what my car had. No engine misfires or related issues have occurred since the fix. The throttle body response is noticeably better too. Just make sure they do a thorough check of all the vacuum lines while they have it apart, catching any worn hoses early can prevent future problems. The air filter was actually still in good shape in my case, but having them inspect it during the repair is worthwhile since you already have symptoms of intake system issues.

Thanks for the advice. I took my Fox to a new mechanic last week after reading these responses. They found a split vacuum line and damaged intake hose, exactly as described. The leak detection test confirmed multiple small air leaks that were killing my fuel economy. Total repair came to 180€, they replaced the cracked hoses, installed new clamps, and did a full inspection of the turbocharger and intake system. The check engine light disappeared right after the fix and that annoying hissing noise is completely gone. The car runs so much better now. Way more responsive when accelerating and getting back to normal fuel consumption. Really glad I caught this early before it could damage the turbocharger. The new mechanic did great work and took time to explain everything they found. For anyone else with similar symptoms, definitely get it checked out quickly. These intake system leaks only get worse if ignored.