Your BOV is probably leaking. Ther reason you cant take the retur tube off is the air that is released from it when it opens is air that the mas has already calculated in the cars A/F ratio.That air needs to be recirculated. When you dump to the atmosphere you are releasing air that the car thinks its getting but its not and that is why it runs like shit. They also have a tendency to leak which also will mess with the tune of the car. In short if you want to dump the BOV to the atmosphere get a VPC,GM maf w/ traslator or a stand alone.