Mercedes R Klasse Hissing Sound and Fuel Economy Problem

Sounds familiar. I had a similar problem with my old BMW: high fuel consumption, hissing, the whole nine yards. It turned out to be a cracked vacuum hose. Think of it like a straw with a hole in it; you're sucking air, but not getting the full amount through the intended path. Did you check all the hoses connected to the intake manifold? A small crack can cause big problems. Is the hissing louder when the engine is cold?

The hissing seems to be constant, regardless of whether the engine is cold or warm. You mentioned the intake manifold hoses. Are they difficult to access? I'm not very experienced with car repairs, so I'm a bit hesitant to start poking around without a better idea of what I'm doing. Could it be something else entirely?

Access can be tricky depending on the car, and honestly, if you're not comfortable, it's best left to a professional. It could be something else, but the symptoms definitely point towards a vacuum leak in the air intake system. Given the age of your R Klasse, the rubber hoses are likely getting brittle. A mechanic will have the tools and experience to properly diagnose and fix it without causing further damage. Plus, they can check for any other potential issues related to the throttle body.

I appreciate you confirming my suspicions. I'll take it to a workshop. I had a look myself and could not locate any issues, so it's better to let someone with experience handle it. They located a leaking vacuum hose and replaced it. The total repair cost was around 100€. Fuel economy is back to normal and the hissing is gone!