G Klasse Hissing Sound and Fuel Economy Problem

A hissing sound when accelerating, poor fuel economy, and a check engine light strongly suggest an air intake leak. Given the mileage, the intake hoses are prime suspects. Visually inspect all hoses from the air filter box to the throttle body, especially the connections. Cracks or splits, particularly on the underside of the hoses, are common. The turbocharger inlet hose should also be inspected. What error code is the check engine light showing?

I have not checked the error code yet. If the leak is after the mass airflow sensor but before the turbo, would that indicate a specific hose? Is there a common failure in the air intake system for this model and engine?

Yes, a leak after the mass airflow sensor but before the turbo points to a problem in that section of the intake system. Common failures on that engine often involve the corrugated hose connecting the air filter housing to the turbo inlet. The heat and pressure degrade the rubber, leading to cracks. Also, check the smaller vacuum lines branching off the main intake tube; they can become brittle. Given the symptoms, I suggest a workshop visit.

Thanks, that confirms my suspicions. I replaced the intake hose. The hissing is gone, fuel economy is back to normal, and the check engine light is off. The repair cost 100€. I appreciate your input.