I have a strange phenomenom with my 147 JTDm 16v:
When accelerating at part load (constant throttle 20-30%) the boost gets very high, sometimes above 1.5 bar, and then instantly it falls down at a given rpm (mostly above 3000 rpm). The result is an unpleasant chance in acceleration while the accelerator pedal is in a constant position. This due to the exhaust backpressure that falls down with the boost, increasing the engine power while injection rate is more or less constant.
Also I noticed if a press the accelerator a bit more while the boost is building up, the boost immediately falls with 0,5-1 bar.
I suspected it was a result of the intake flaps and/or a sticking VNT mechanism.
So I made some logs, and to my suprise it doesn't look like that:
As you can see the flaps are fully open already at +/- 2200 rpm. And, a sticking VNT mechanism would cause raising boost with decreasing VNT %.
It looks like it is really programmed like this, because the VNT% also makes a sudden fall that results in the actual boost fall.
This only occurs at part load, at full load there are no problems at all. Boost is nicely kept constant through the rev range.
Anybody also have this experience? Or know what this is and why ?