GLK Fuel Consumption and Rich Running Condition

Sounds like a rich mixture issue. I had something similar with my GLK. Have you checked the intake manifold for leaks? Also, how old is your oxygen sensor? A faulty one can definitely cause the engine to run rich because it messes with the fuel/air ratio. When were the spark plugs last changed?

Thanks for the quick reply! I haven't checked the intake manifold myself, not really sure how to. The oxygen sensor, as far as I know, hasn't been replaced since I bought the car a few years ago. I also don't know about the spark plugs. Could all of that really cause such a big hit to my fuel economy and the rough running?

Okay, the oxygen sensor is a strong suspect then, especially if it's original. A bad sensor can send incorrect data to the engine control unit, causing it to dump too much fuel. The intake manifold is a bit trickier to diagnose without tools and experience. Honestly, with those symptoms – reduced power, check engine light, rough running, increased fuel consumption, jerking – and the potential for multiple causes, I'd recommend taking it to a workshop. They can properly diagnose it with the right equipment. It could save you money in the long run by avoiding unnecessary parts replacements.

You were spot on! Took it to a trusted mechanic and they confirmed a defective injection valve. Replaced all of them and the car runs like new. Total cost was around 515€. Thanks for the help!