Mercedes GLE diesel engine light and rough idling

That sounds like a fuel issue alright. Rough idling and reduced power, classic signs. Have you noticed any change in your fuel economy recently? A bad fuel volume regulator could definitely mess with your fuel injection and fuel efficiency. Maybe check for any obvious fuel leaks first before diving in too deep. What about any unusual noises from the engine?

Fuel economy has been a bit off, yeah. No visible leaks, checked that already. No unusual noises either. You mentioned fuel injection, could that be affected even if the injectors themselves are fine?

Absolutely. If the fuel pressure isn't right because of a faulty regulator, the injectors won't get the fuel they need at the right pressure. It throws off the whole system. Given the symptoms, and that you've checked the basics, a faulty high pressure pump and volume regulator are the most likely cause. I'd say it's time to get it checked out by a workshop.

Thanks for confirming my suspicions. Took it to the shop; you were spot on. Fuel volume regulator was indeed defective, caused by a failing high pressure pump. Repair cost 1510€. At least now I know what to expect regarding car maintenance on this thing.