When I drive on the European highway, set my DISTRONIC PLUS to >90 mph or more and encounter a long uphill section, the car struggles to maintain the set speed in the 9th gear and eventually slows down to <80 mph without downshifting.

When I downshift manually, there's no problem and the car maintans/returns to the preset speed. I suspect this won't be a problem for larger engines because they have enough power to maintain the speed in the 9th.

Does this happen to anyone else? Shouldn't the car be able to downshift automatically when it starts slowing down? It has no problem doing it for reasons other than lack of power...

 

Hmmm, did it felt like the car is lugging the engine by any chance?

 

At this speed it's still above 2000 rpm, so there's no vibration or anything, it's just going full throttle and decelerates because it doesn't have enough power. As soon as the slope eases it starts to pick up speed again.

I also forgot to mention that it's been this way since the car was new and there are no stored faults or anything - authorized dealer inspected the car, updated all software, downloaded data from various ECUs.. all seems OK.

 

It does seem like it is normal behaviour for your particular vehicle, I never came across something like this, I had C300 loaners here and never had such issue, unfortunately we don't have C180 here so I am not quite sure. I am sorry that I wasn't able to help you. Hopefully someone else post here and comment about it.

What I could suggest doing is to ask your dealership do a manual transmission adaptation depending on the speed of the vehicle and the current RPM. I hope they are able to assist you with that.