You'll need to install and tune a fuel controller which is able to account for the fact that air that was measured isn't actually making it into the engine. On a stock system... you just have to deal with it... some people get lucky and the car remains somewhat drivable in town. Oh, and then there's the back firing... what fun!
The thing is that the ECU gets information from the airflow meter about how much air has been taken into the system. The stock BPV removes excess boost pressure by rerouting the air to further upstream where it isn't compressed but is still after the meter. When you vent to atmosphere the ECU still thinks there is a TON more air in the system than there is and keeps providing a lot more fuel than needed during those couple seconds... thus: backfire and stall problems.
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