My other car is a MINI!!!!
basically, it's a system that uses the vacuum in the inlet manifold at idle to draw gasses out of the crank case, then, under boost, there is a check valve to stop the boost going back down the system.
Sadly, on the AGU, the check valve is joined to the inlet manifold by a 6'' bit of pipe, with 2 90 degree bends in it.
It would appear, that the oilly vapour that is drawn through this tube eats the rubber over time, and they almost always split.
Since finding the fault on my old AGU about 2 years ago, I've only found one AGU that doesn't have a split, yet about 10 that do! So it's a fair guess to assume it'll be split on yours. it's almost always overlooked!
I fixed mine, and it made a huge difference, and my fuel economy went up massively too.
if it's leaking out under boost, the turbo will still be drawing the air in, so the MAF will see the high flows, and fuel for all that's been drawn in. If say 10% of that (for example) is then leaking out under pressure, you'll be having 10% too much fuel!