[lico]

Can't not be sure 100% but diesel engine of the last year start pushing a bit after the 1500 rpm, so it can be that below that point turbo act more as a brake to the exhaust gas making the entire cycle less efficient, hope it's not an oil issues, that engine it's suppose to have a variable oil pump.

my dream car AX 1.5 D

edit

i have noted, with great disappointment, that in modern engine with drive by wire system, the load to the accelerator it can be misunderstood by the ecu: when i drive the ford focus 1.6 tdci of my brother on a plain @ x speed on flat surface, and after a while the road start a light slope, if I keep the same load on the accelerator the car won't slow down instead it try to keep same speed increasing the mpg...it sucks
so to keep the rpm in real maybe the ecu need to put moore diesel x cycle