Mercedes CLK Hissing Sound and Fuel Economy Issue

A hissing sound on acceleration, poor fuel economy, and a check engine light suggest an air intake leak. The intake manifold is a likely area. Have you done a leak detection test? Engine misfire can occur as unmetered air enters. Confirm the leak's location before replacing parts.

A leak detection test? How do I do that? If it's the intake manifold, is that a hard repair? I hope it's just a hose I can replace.

A smoke test is the best leak detection method, usually done by a workshop. Visual inspection can show cracks or disconnections, especially in intake hoses. A damaged intake hose is likely, but intake manifold leaks can happen. The engine misfire and fuel economy point to a big leak. I suggest a professional diagnosis to avoid problems.

I took it to a mechanic. It was a split in an intake hose. Total cost was about 100€ for the hose and labor. Fuel economy is normal, and the check engine light is off. Thanks for the help!