Peugeot 406 high fuel consumption hissing sound

Hissing under acceleration and a check engine light on your 406? Sounds like you're on the right track with the intake leak suspicion! Higher fuel consumption would definitely point in that direction too. Have you tried visually inspecting the intake hoses for any cracks or splits? A good starting point is to check the vacuum hoses connected to the intake manifold – those can get brittle with age. Any luck with leak detection spray?

Thanks for your response. I did a visual inspection, but everything looked fine at first glance. Maybe I missed something. Where exactly should I be focusing my attention regarding the vacuum hoses? And where to buy that leak detection spray from, specifically?

Alright, if a quick look didn't reveal the culprit, try getting a bit more hands-on. The vacuum lines running off the intake manifold are prime suspects, especially the ones hidden underneath. Wiggle them a bit and see if you hear any change in the engine sound (wear gloves!). As for the spray, any auto parts shop should have it – usually sold as intake cleaner. Is it worth pulling it apart and replacing the vacuum lines to try to sort it out, or shall we go to a workshop right away?

Thanks a lot for the advice. I checked the hoses, and it turned out to be a split in one of the smaller vacuum hoses connected to the intake manifold, just like you suggested. I replaced the hose, cleared the error code, and everything seems to be running smoothly now. Repair cost about 100€. Fuel economy seems to be back to normal. Appreciate the help!