2l tdi 140 bkp poor mpg....

[seb.g]

My parents have had a 2008 passat 2.0 tdi bkp for the last few years and
ever since they have had it it has never really given very good mpg figures at all.
The car all ran fine etc, and i have asked the question a few times before
as to why it was happening, tried lots of the suggestions but to no avail,
now every now and again the car when under load in 4th gear or above
would go into limp mode as it was 'overboosting'.