Mercedes GLS Diesel Intake Manifold Issue

Those symptoms do align with a faulty intake manifold flap control. The hissing sound suggests a vacuum leak, possibly related to the actuator or the manifold itself. The flap control manages airflow, which is critical for optimal engine performance. Has any diagnostic work been performed to confirm the specific fault code?

I haven't done any serious diagnostics myself. I'm vaguely familiar with the intake system and know flap control is important, but I am unsure to what extent. What tests could I perform to know for sure it is the switch?

Given the complexity and potential for misdiagnosis, especially with modern engine management systems, DIY testing can be tricky. While you could visually inspect the actuator arm for movement with engine revs, or check vacuum lines for leaks, confirming the root cause definitively requires specialized tools and expertise. Based on your symptoms, the intake manifold flap control switch is likely the culprit, but I'd strongly advise taking it to a reputable workshop. Continuing to drive it with this issue could cause additional damage.

Thanks, that's what I was afraid of. Dropped it off at the shop today. They confirmed it's the intake manifold flap control and quoted me 465€ for the repair. Hopefully, this fixes the reduced power and other issues. Appreciate your help!