VW Lupo Diesel Loss: Turbo vs Vacuum Leak Mystery

I dealt with nearly identical symptoms on my 2001 VW Polo diesel last year. Having some experience with diesel engines, these symptoms immediately pointed to an air intake system issue. The reduced power and increased fuel economy, combined with that distinctive hissing noise, matched my situation perfectly. When I took it to the workshop, they found a cracked vacuum hose near the turbocharger, which was causing the air intake problems and triggering the check engine light. The fix was straightforward, replacing the damaged intake hose. Parts and labor came to 100€. Since the repair, engine performance returned to normal and fuel consumption improved significantly. To help diagnose your case more precisely, could you share: Current mileage on your Lupo, Whether the hissing sound changes with engine RPM, If you notice any black smoke from the exhaust, When the check engine light first appeared, Any other unusual sounds or performance changes These details would help determine if your issue matches what I experienced with the intake system or if it might be something else in the air intake or turbocharger assembly.

Last service was done at 88000km, just before these issues started. The hissing sound definitely changes with engine RPM, getting louder during acceleration. Noticed some black smoke from the exhaust, especially when accelerating hard. The check engine light came on about a week ago, right when the fuel economy started getting worse. I checked the air filter and it looks pretty clean since it was replaced during the last service. I tried looking for obvious vacuum hose leaks around the throttle body but couldn't spot anything obvious. Would a smoke test help locate the leak if that's what it is? The turbo seems to still build boost, but not as strongly as before.

Thanks for those details, they match exactly what I experienced. The symptoms on your Lupo are nearly identical to what happened with my Polo, particularly the RPM-dependent hissing and black smoke during acceleration. In my case, even though the air filter was clean, the issue was a hairline crack in the intake manifold that was hard to spot visually. A smoke test would definitely help locate the leak, thats actually how my mechanic found my issue, as it was hidden behind the throttle body assembly. The black smoke and reduced turbocharger performance strongly suggest an air leak between the turbocharger and intake manifold. When this happens, the engine computer tries to compensate by injecting more fuel, leading to the poor economy and smoke. Before spending money at a workshop, you might want to check: 1. All clamps around the turbocharger are tight 2. The rubber boots connecting the air intake components for any small splits 3. The intake manifold gasket for any signs of failure A mechanic with a smoke tester can pinpoint the exact location quickly. For my repair, the smoke test cost 40€, and it saved time hunting down the leak. The complete fix, including parts and labor, ended up around 100€ for the intake manifold gasket replacement.

Thanks for the detailed troubleshooting suggestions. The symptoms you described with your Polo match my Lupo issues exactly, especially the connection between the hissing noise and turbocharger performance. I just did another thorough check following your suggestions. Found that one of the clamps near the turbocharger feels a bit loose, and theres a small split developing in one of the rubber boots, probably the source of my air intake problems. The throttle body area looks okay though. Think I will book it in for that smoke test to confirm the leak detection before getting the repairs done. The 140€ total cost you mentioned for diagnosis and repair seems reasonable, especially considering how much extra I am spending on fuel right now due to the poor fuel economy. Will give my regular workshop a call tomorrow and mention these specific symptoms and the likely air intake system issue. Having this information should help them diagnose it faster. Really appreciate you sharing your experience, saved me a lot of guesswork.