Citroen Ccrosser Fuel Consumption and Hissing

Sounds familiar. I had similar symptoms with my old Saab 9-3. Check for leaks in the intake hose, especially around where it connects to the throttle body. A cracked vacuum hose can cause that hissing sound and throw off the fuel mixture.

Thanks! Where exactly should I be looking? Any tips for leak detection?

Focus on the hose between the air filter and the engine. Feel around for cracks, especially on the underside. If you can't find anything visually, try spraying some soapy water on the hose while the engine is running - bubbles will form if there's a leak. Honestly, with those symptoms and age, just get it to a workshop, it may be more than the hose itself

Yep, it was the intake hose. Cost me 100€ at the workshop. Thanks for the pointers!