Seat Ibiza Fuel Economy and Hissing Sound

Regarding your Seat Ibiza from 2001, those symptoms do align with a potential air intake leak. The hissing sound, increased fuel consumption, and check engine light are typical indicators. The air intake system is designed to deliver the correct amount of air to the engine for optimal combustion. A leak can disrupt this balance.

Thanks for the quick reply! I don't have an OBD2 scanner to read the error code. Where exactly should I look for the vacuum hose and air intake hose connections? I'm not super familiar with the engine layout, but I'm willing to take a look myself before taking it to a mechanic.

Specifically, when you say the check engine light is on, do you know what error code is being reported? Also, have you visually inspected the vacuum hose connections around the intake manifold and throttle body? A loose or cracked vacuum hose can cause similar issues. Given you don't have an OBD2 scanner, I recommend a careful visual inspection. Follow the air intake from the air filter box to the throttle body and then to the intake manifold. Pay close attention to any rubber or plastic hoses connecting these components, especially the vacuum hose lines. Cracks or loose connections are what you're looking for. Since you hear hissing when accelerating, the intake hose leaking is highly likely. If you're not comfortable with this, I'd advise taking it to a trusted mechanic. While these issues can be DIY-repaired, it is better to be sure.

You were spot on! I checked the air intake hose like you said, and there was a big crack in it. I replaced it myself, only cost me 100€. The hissing is gone, the engine light turned off after a short drive, and my fuel economy is back to normal. Thanks a million for your help!