GLK Diesel Intake Manifold Issue

That sounds frustrating. I had a similar problem with my old Vauxhall Astra diesel a few years back: reduced power, engine light, the works. It turned out to be the swirl flaps in the intake manifold. Are your intake manifold flaps the same thing? They were sticking and not opening or closing properly, which messed with the air/fuel mixture. Does the hissing sound like it's coming from the engine area? If the flaps are stuck partially open, that could cause a vacuum leak, which could explain the hissing, rough running, and reduced power. Has the engine light come on yet? If so, have you had the error codes read? That's the first thing I'd do. It will give you a much better idea of what's going on and might confirm your suspicion about the manifold flap. I hope this helps.

Thanks for the input. You mentioned the swirl flaps in your Vauxhall. Are they part of the intake manifold on that car, or are they a separate component? I'm just trying to understand if the issue you had is directly comparable to what I'm experiencing. The hissing definitely seems to be coming from the engine area, possibly around the intake manifold itself. The engine light hasn't come on yet, but I have a feeling it's only a matter of time.

On my Astra, the swirl flaps were integrated into the intake manifold. Basically, the manifold had these flaps inside that were supposed to help control the airflow into the cylinders, especially at lower engine speeds. When they got gunked up with carbon deposits, they started to stick, causing problems. Given that the hissing seems to be coming from the manifold area, and you're experiencing similar symptoms to what I had, it definitely sounds like it could be a manifold issue. Considering the age and mileage of your GLK, I would suggest taking it to a workshop to get a proper diagnosis and to avoid further damage.

Thanks again for the detailed replies. I took it to a workshop, and they confirmed it was the intake manifold flap control. Defective switch as suspected. They replaced it, and the total cost came to 465€. The engine performance is back to normal, the rough running and jerking are gone, and the hissing is no more. Appreciate the advice!