Mercedes ML engine power loss rough running and high fuel consumption

Those symptoms certainly sound concerning. A rich mixture could definitely be the culprit, and a faulty injector is a prime suspect. Before jumping to conclusions, have you checked for any obvious vacuum leaks? Also, when was the last time you replaced your air filter? A clogged air filter can restrict airflow and cause a rich running condition. What diagnostic codes is the check engine light throwing? Knowing those codes would help narrow down the possibilities significantly. It may also be helpful to know what your recent fuel economy has been like.

I have not checked for vacuum leaks yet. The air filter was replaced approximately 20000 kilometers ago. I do not have the means to retrieve the diagnostic codes myself. How might vacuum leaks affect the air-fuel ratio?

Okay, 20000 km on the air filter is probably not the main issue, but still worth a look. Vacuum leaks let extra air into the engine after the mass airflow sensor. This unmetered air leans out the mixture in some cylinders, but the engine control unit (ECU) tries to compensate by adding more fuel overall, resulting in a rich mixture in other cylinders. Since you do not have the ability to check the error codes yourself, it sounds like it is time to take it into the shop. It could be a faulty injector like you suspect, but without the codes it is just guessing. A proper diagnosis will save you money and time in the long run.

I took the vehicle to a trusted workshop, following your recommendation. The diagnosis confirmed a defective fuel injector. The total cost for the replacement and associated labor amounted to 515€. The engine now operates smoothly, and the fuel economy has returned to its normal level.