DTS Hissing Sound Points to Vacuum Leak

I experienced something very similar with my 2005 STS. The symptoms you describe match exactly what I dealt with, poor fuel economy, check engine light, and that distinct hissing noise under acceleration. After some basic troubleshooting, my mechanic found a split vacuum hose near the intake manifold. The repair was straightforward and cost 100€ including parts and labor. The difference in performance after fixing it was immediately noticeable, the engine ran smoother and the fuel consumption returned to normal. With some experience working on similar engines, I can say these symptoms often point to an air leak in the intake system. The hissing sound is typically caused by air being pulled in through a crack or loose connection, which also explains the increased fuel consumption as the engine management system tries to compensate. Could you provide some additional details that would help diagnose this: Have you noticed any rough idling? Did the check engine code get scanned? Is the air filter maintenance up to date? Any recent work done on the turbocharger or intake system? This information would help confirm if your issue matches what I encountered.

Thanks for the helpful info on the vacuum leak possibility. My mileage is at 189961 KM and last service was due to throttle body cleaning. I ran a diagnostic scan and got code P0171 which points to a lean fuel mixture. After checking under the hood, I found the rubber intake hoses look pretty worn, especially near the manifold connection points. The engine definitely idles rough when cold and smooths out after warming up. No recent work on the intake system besides basic maintenance. Going to do a proper leak detection test with a smoke machine before deciding on repairs. The increased fuel consumption and that hissing sound make me think you might be right about the vacuum hose theory.

This definitely confirms my initial suspicions. The P0171 code matches exactly what I saw with my STS before finding the vacuum leak. After seeing your mileage and the fact you had recent throttle body work, I can almost guarantee were dealing with the same issue. The rough cold idle is another telling sign. When I had my vacuum leak, the engine would struggle until reaching operating temperature, just like youre describing. The worn rubber hoses near the intake manifold are likely the culprits, these parts tend to deteriorate around this mileage. Smart move planning a smoke test for leak detection. Thats actually what helped pinpoint the exact location in my case. While youre at it, have them check all vacuum lines connected to the intake manifold. My repair revealed multiple weak points that needed addressing. The recent throttle body cleaning could have inadvertently stressed some of those old rubber connections. Once you fix the leak, make sure to check the air filter too, these issues sometimes go hand in hand. Quick tip from my experience: when replacing vacuum lines, consider doing all of them in that area. It saved me from having to fix another leak a few months later.

Thanks for all the details about your similar experience. After reviewing everything, I think I'll definitely proceed with the smoke test to pinpoint any leaks. The P0171 code and symptoms match up perfectly with what you encountered, especially the way the engine misfire acts up during cold starts and the connection to recent throttle body maintenance. Going to have my mechanic do a thorough check of the entire intake manifold system and replace all the vacuum lines while they're at it. Makes sense to do them all at once given the mileage. The poor fuel economy and that constant hissing sound during acceleration are getting worse, so better to address it soon before it causes additional problems. Will update once I get the smoke test results and repairs completed. Really helpful to know this was a relatively straightforward fix in your case. Hoping my repair bill stays around that 100€ mark too.