Elysion hissing sound and fuel economy drop

Hissing and fuel economy dropping rings a bell. I had something similar on my Elysion, though I've got a slightly earlier model. Have you checked the intake hose that runs from the air filter box to the throttle body? It's a corrugated plastic thing, and they're known to crack, especially near the clamps. Did you also check the hose connecting the turbocharger?

Throttle body hose, huh? I was eyeballing that, but didn't see anything obvious. You reckon it's worth tearing into it for a closer look? What did your cracked hose look like? Was it an obvious split, or more subtle?

Definitely give it a closer look. Mine wasn't immediately obvious – a hairline crack on the underside of the hose that opened up under pressure. That would explain the hissing. If it is cracked, you might want to just bite the bullet and take it to a decent workshop to get it smoke tested and properly fixed; they should also check for any error codes related to the misfire.

That's exactly what I needed to hear. Took it to my mechanic, smoke test confirmed a split in the intake hose just like you said. New hose fitted, cost me about 100€, and she's purring like a kitten again. Fuel economy's back to normal, and the check engine light is finally gone. Thanks for the help!