Seat Arosa Petrol Engine Hissing Sound and Poor Fuel Economy

Sounds like a vacuum leak to me. I had a similar issue with my old Vauxhall Astra. It was like listening to a kettle whistle whenever I put my foot down. Check all your vacuum hoses, especially the one that goes to the intake manifold and the throttle body. A split in the hose can cause higher fuel consumption and trigger the check engine light. It's often a cheap fix, but a pain to diagnose sometimes

That's interesting. You mentioned checking the vacuum hose to the intake manifold and throttle body. Is there a particular spot on those hoses that tends to fail I'm just trying to narrow down the search, as there are a few hoses under the hood of my Arosa, and I am not sure where exactly to look. Also, would a leak there affect the turbocharger in any way, or is that a separate system

Yeah, usually it's where the hose bends or connects to something. Think of it like bending a straw too much – it'll crack at the flex point. As for the turbocharger, yours doesn't have one, so you don't have to worry about that. But honestly, if you're not 100% confident, it might be worth getting a mechanic to take a quick look. They can smoke test it to find the leak in seconds

You were spot on I checked the vacuum hose that connects to the intake manifold, and there it was a big crack. Replaced the hose, and the hissing is gone, the check engine light is off, and I swear the fuel economy is already better. Cost me around 100€ for the hose and a bit of fiddling. Thanks a million for your help