i30 Hissing Sound and Fuel Consumption

I had a similar issue with my i30, although with different mileage. Higher fuel consumption and a hissing sound are classic symptoms of an air intake leak. Check all your hoses connected to the intake manifold and air filter. Sometimes it can be a split or crack that's hard to see unless you really get in there and inspect them. Did you check the engine code from the check engine light?

Thanks for the tip! I'll give those hoses a really good look. I checked the error codes, and it gave me a P0171 code, which means 'System Too Lean (Bank 1)'. I realize that it's pointing towards a lean mixture, meaning too much air and not enough fuel, that's why I'm leaning towards a leak somewhere. Could you tell me what fixed the problem for you back then?

That code confirms a leak after the mass airflow sensor. In my case, it turned out to be the intake hose connecting to the turbocharger - it had a small crack on the underside that was almost impossible to see without removing it. I'd recommend taking it to a mechanic with smoke machine, which will help to confirm for sure where the leak is located and avoid unnecessary spending.

You were spot on! I took it to a local shop, and they confirmed it was indeed a cracked intake hose. The smoke test made the leak immediately obvious, and they replaced the hose for about 100€. The hissing is gone, the check engine light is off, and the fuel consumption seems back to normal. Thanks a bunch for your help!