Right - I'm trying to get to the bottom of The Tractor's love for diesel (as per before - never cracked 30mpg at 70-75mph on the 408 miles home and now returning mid 20's with mixed driving). Apparently it should have managed 35mpg+ on the motorway and around 30mpg in our usual driving. Published MPG figures say that it should return 5mpg less than my old 335d across all conditions. It's currently -10mpg.
I finally remembered how to get into the hidden OBC menu and then the coolant temperature display. I went off up the road and gave it some (the engine wasn't cold), and sure enough the temperature climbed from 70 degrees up to a peak of 91 (which I understand is roughly where it should sit).
"Bugger" I think, "it's not a thermostat stuck open then"
But then I turned around and headed home at a more sedate pace - it's largely down hill but throttle application still needed. The temperature then plummeted to the mid-early 70's and stayed there.
Thinking logically, is that not textbook open thermostat? Unimpeded coolant capable of keeping the engine cool enough under maximum load, and then lower load situations resulting in way low temperatures?