Power loss on F800GS
I'm currently in Bolivia with what sounds like an identical problem on my F800GS.
It first occured in Argentina over a week ago, 30 degrees midday temperature with 30km remaining showing on the OBC. I filled with fuel and all was good until a couple of hours later (with > 180km remaing on OBC). It got worse so I stopped for the day.
The next morning the problem did not occur (it was only 20 degrees). However it re-occured later in the day as it rose to 30 degrees, again with plenty of fuel in the tank).
This is all with easy cruising at 100km/hr.
There's a lot of stuff on ADvRider & other forums. I'm guessing that the fuel pump and/or fuel pump electronics is giving up at high temperatures (the pump is cooled by the fuel in the tank), so I'm carrying 4 litres of reserve fuel and continually refuelling, which seems to be working. Also, I'm now up on the altiplano at 2000-4000m so it's a bit cooler.
I too am nowhere near a BMW dealer. I emailed all five dealers in Argentina a week ago asking if they had replacement fuel pump parts, but guess what - none have bothered to reply. I spoke to a BMW service guy in Sydney, but he hadn't come across this before.
I'm staying put here for 2 or 3 weeks. I'm thinking of fitting a small 12V fan beneath the seat which could cool the heatsink on the fuel pump electronics. However, I don't reckon this will help if it's the pump that's faulty.
Let me know if you find a solution.