Mercedes GLC Diesel rough idle and high fuel consumption

A defective throttle valve can cause those symptoms. Higher fuel consumption and poor starting are common indicators. Have you checked for any error codes using an OBD2 scanner? Also, inspect the throttle body for excessive carbon buildup. A malfunctioning accelerator pedal module could also be the culprit. In my case with my GLC, it was a faulty throttle valve.

I don't have a scanner. Where is the throttle body located, and is throttle cleaning something I can do myself, or is it better left to a pro? If it was the throttle valve how much did that cost to get fixed?

The throttle body is typically located between the air intake and the intake manifold. While throttle cleaning is possible DIY, improper cleaning can damage sensitive components. Considering the potential for complications and the need for specialized tools, I would recommend professional diagnosis and repair. I suggest taking it to a workshop. For me, the repair cost 205€.

Took it to the shop, and you were spot on. It was a defective throttle valve, just like you said. Got it replaced and everything is running smoothly again. I appreciate the help!