Well unfortunately my 2.0TDi BKD under-boost issue that I thought I had completely fixed has reared it's head again.
It is a lot better since I replaced a damaged boost o-ring, but it dropped an under boost code yesterday.
In my previous testing I have confirmed that the VNT actuator is OK and it seems to be adjusted correctly as it starts to move at 5"Hg and is completely open at 20"Hg of vacuum - so that all looks OK.
I'm interested in knowing how the boost control valve in the solenoid block works, does it energise to vacuum the actuator, or is it energised to bleed off vacuum ? I'm wondering if a lazy valve in the solenoid block could cause an occasional under-boost code.
They are £160 incl the dreaded so I'd rather not buy one if it won't eliminate the issue.