S660 Hissing Sound and Fuel Consumption

That sounds like a classic intake leak. The hissing is a dead giveaway. Have you tried visually inspecting all the intake hoses, especially the one between the air filter box and the throttle body? Those can crack over time, especially with heat. Also, has the car been throwing any specific OBDII codes when the check engine light comes on? That would give us a better idea of what the ECU is seeing. Could be running lean, which messes with fuel economy.

I did a visual inspection of the intake hoses, but I didn't see any breaks. I didn't record the OBDII codes before clearing the check engine light. Might a leak in the intake manifold itself cause such symptoms, or is that improbable?

Okay, no codes recorded. An intake manifold leak is possible, though less common than a simple hose issue. Manifold leaks can be tricky to spot. If the hissing is louder near the manifold, that's a clue. Honestly, without codes or being able to physically inspect it, it's tough to say. Given the symptoms, I’d advise a reputable workshop to perform a smoke test. That'll pinpoint the leak pretty quickly. Better to catch it early before it causes any misfires or further fuel economy woes.

Following your recommendation, I brought the vehicle to a mechanic who performed a smoke test. The test revealed a leak in the intake hose, as initially suspected. The hose was replaced at a cost of about 100€. The issue has been resolved. Thanks again for your assistance.