Subaru Diesel: Hissing Intake Points to Vacuum Leak

Had a similar issue with my 2003 Subaru Trezia diesel. As someone with moderate experience working on cars, the symptoms you describe match exactly what I encountered, the hissing noise and increased fuel consumption were the main indicators. The check engine light led me to have it properly diagnosed. Turned out the intake manifold had a crack and several vacuum hoses were deteriorated. The workshop performed a smoke test to identify the exact location of the air intake leak. The repair involved replacing the damaged vacuum hoses and sealing the intake manifold properly. Total cost was 100€ for parts and labor, which included: Diagnostic testing, Vacuum hose replacement, Intake manifold inspection and sealing The repair resolved both the hissing noise and fuel consumption issues completely. The check engine light cleared after the fix. To provide more specific advice for your case, could you share: current mileage on your Justy, How long the symptoms have been present, whether the noise occurs at specific RPM ranges, if any performance issues accompany the symptoms These details would help determine if your issue matches what I experienced or might be something different.

Thanks for the detailed response. Last service was 2 months ago at 134104 KM. The vacuum hose and leak detection test seems like the right approach given our similar symptoms. I noticed the hissing occurs mostly above 2000 RPM. The fuel economy drop started about 3 weeks ago, getting steadily worse. The air filter was replaced during my last service, so I doubt thats causing the increased fuel consumption. Given the age of our cars, deteriorated vacuum lines make sense. Will book a diagnostic test to confirm the exact issue.

Thanks for providing those additional details. Your mileage and symptom timeline match closely with what I experienced with my Trezia. The 2000 RPM threshold for the hissing is particularly telling, that is exactly when my leak became most noticeable due to increased turbocharger boost pressure. The recent air filter replacement actually helps narrow things down. Since that is already new, the leak detection test should focus on the section between the turbocharger and intake manifold, where the highest pressures occur. In my case, the vacuum hoses had become brittle exactly in this area. A good mechanic should: 1. Perform a smoke test for proper leak detection 2. Check all vacuum hoses connected to the intake manifold 3. Inspect the turbocharger boost connections 4. Test the intake manifold gaskets The steadily worsening fuel consumption suggests a progressive failure, similar to my situation where the vacuum hoses gradually deteriorated rather than failing suddenly. Getting it checked soon is wise before the leak potentially affects other components.

Thanks for this thorough info. The symptoms and repair path you described match my situation perfectly. Since my hissing also starts at 2000 RPM and the fuel consumption has been getting progressively worse, I feel confident booking the diagnostic test. Given that my air filter is new but the vacuum hoses are original, focusing the inspection on the turbo-to-intake manifold section makes complete sense. The check engine light and hissing noise pattern really point toward vacuum system issues rather than throttle body problems. Will schedule the smoke test and full inspection of the intake manifold area next week. The 100€ repair cost you mentioned seems reasonable for the work involved. Good to know this resolved similar symptoms completely in your case. Really appreciate the detailed breakdown of the testing and repair process, helps me know exactly what to expect at the workshop.