iQ Hissing Sound Poor Fuel Economy

Hey there, I have an iQ as well, though not the same year. Those symptoms do sound familiar. A hissing sound on acceleration usually points to a vacuum leak in the air intake system. The higher fuel consumption and check engine light would also make sense. Have you had a look at the intake hose, the one going from the air filter box to the throttle body, for any obvious cracks or splits? That's a common spot.

Thanks for the quick response! I'll check the intake hose. Is it easy to spot the cracks? I'm not a mechanic but I'm reasonably handy.

Sometimes the cracks are obvious, other times they're small and hidden, especially on the underside of the hose. Try wiggling the hose while the engine is running (carefully!) and listen for changes in the hissing sound. If you're unsure, take it to a workshop and let them do a smoke test to find the leaks.

Took it to a shop as you suggested. It turned out to be a split intake hose, exactly as you described. They replaced it for 100€. Fuel economy seems back to normal now. Thanks a lot for your help!