The brief
The GLC300 had developed misfires, a rough idle, a loss of power under acceleration, the occasional intermittent stall, and a whine from the engine area, with a check engine light on the dash. He brought it in. That combination points at the high-pressure fuel pump. On a direct-injection engine like this, that pump takes the fuel from the low-pressure side and pressurises it to the very high pressure the injectors need to spray it into the cylinders. When it starts failing it can't hold that pressure steadily, so the injectors don't get a consistent supply, which is the misfires, the rough idle, the power loss and the stalls, and the whine is the pump itself struggling. A high-pressure pump on its way out only gets worse, and the next step is a no-start.
The diagnosis
Diagnostics confirmed the fuel rail pressure wasn't being held at spec under load, and the codes pointed at the high-pressure fuel side, with the whine lining up with wear in the pump. The low-pressure pump and the injectors checked out, so the high-pressure pump was the fault. That's a replacement. The high-pressure pump is a precision sealed unit, you don't rebuild it, and one that's losing pressure only loses more, so it was getting changed.
The work
The intake side was cleared to reach the high-pressure pump, the old pump removed, and a new genuine Mercedes-spec high-pressure fuel pump fitted with fresh seals, the fuel lines reconnected. Then the system was primed, the rail pressure read on the scanner across idle and load to confirm it was holding spec, and the stored fault codes cleared. A road test confirmed the misfires, the rough idle and the power loss were gone and the whine was gone.
The outcome
Steady idle, full power under acceleration, no misfires, no stalling, no whine, and the check engine light out after a drive cycle. The GLC300 went home running cleanly. A failing high-pressure fuel pump makes a healthy engine run terribly and eventually strands you, so changing it and confirming the rail pressure put the running properly right.