I believe I asked the same question before, but never got an answer...maybe cos no one knows the 3.0 well enough
I shall begin by explaining what was "wrong" with my old 3.0CDTi with 160k+ miles.
That thing (let her rest in peace wherever she might be ) had a NEW turbo fitted, NEW EGR valve, injectors with 50k miles, cleaned boost sensor, intercooler and hoses in order,...decat...
However, it was still a bit down on power. I suspected a bad MAF sensor, but back to that later...
The main problem was that it smoked whenever I revved it in neutral. It was so bad that for a moment I couldn't see behind, BUT if I redlined it in any gear it barely put out any smoke.
I then cleaned what I thought was the culprit of the smoke and drop in performance, but it only helped with the power for a while, then it got sluggish again after a few miles...shot sensor I suppose.
Now to my current car. Mileage around 120k, original injectors, original EGR, original fuel pump, turbo with around 30k, intercooler and hoses in order, with catalytic converter still fitted,...
It pulls like a tank, with no apparent power loss anywhere in the rev range apart from a slight delay when I accelerate through 2600rpm.
Despite everything I tried with this engine, Millers or 2-stroke oil, it still smokes if revved in neutral. It won't smoke if ragged through the gears, solved that with the additives and an "Italian tune-up" in Germany.
So, both cars behave the same way but each has different history of repairs.
Alright chaps, need your suggestions now, because every garage I went to was stumped by this