SM5 Hissing and Fuel Consumption

Hissing and poor fuel economy? Sounds familiar. I had similar symptoms on my SM5. Have you checked the intake hose between the air filter and the turbocharger? Those hoses are known to crack with age. Makes a racket when the turbo spools up and it messes with the air/fuel mixture. Is your engine misfiring at all?

Thank you for the suggestion. I'll inspect the intake hose as you advised but I did not observe any misfiring. If there a leak, does it always trigger the Check Engine light?

A leak big enough will definitely trigger the check engine light because it throws off the oxygen sensor readings. And yeah, my intake hose had a split on the underside, I discovered it only after really feeling around. If you can't diagnose it yourself, take it to a workshop. It's a cheap fix, but a pain to track down sometimes.

Thank you for the useful advice. I inspected the intake hose as you suggested, and indeed, there was a significant crack. Replacing it resolved the hissing sound, and the fuel consumption has improved. After replacing the leaking intake hose, the check engine light turned off by itself after a while. The repair cost amounted to around 100€. Thanks again!