hi mate, in case you didn't already know the BOV will act basically as an air leak in the induction system. The MAF will meter exactly how much air goes in and what goes in should basically be combusted with fuel and that's it. Even if the DV opens, the air that is exhausted goes back into the inlet for another lap of the induction system until it is eventually burnt. Whats going to happen is that the MAF will see a certain amount of air and tell the fuel injectors exactly what to add to it to get the mixture right. But then you are going to let some of that air escape which will cause a rich condition. It may also suck some air in when under full vacuum depending on how strong the BOV spring is causing a lean condition at those times. End result is that your fuel trims will be all over the shop as the O2 sensor will be seeing mixtures that don't square up with what it expects and so the ECU will be putting in fuel offsets/trims to compensate for the average of what is going wrong. If you are on a mafless tune, different story, but with a MAF a BOV aint a good idea if you want it to be running tip top. You probably can block the vent with a GFB plug and turn it into a DV again. Just didn't want you to do it without eyes wide open cos then you might chase other things if the car feels a bit wrong with it is all.
