[Chooch84]

The stock ECU can only take into account the air that has gone past the AMM, any that has been vented to atmosphere isn't metered by the ECU in any way so it can't really make a decision based on something it doesn't know. The AMM has told the ECU that more air has gone past it hence the leaning out. MAP sensors are generally used by themselves or in combination with a MAF to overcome this issue (modern OEM ECU's), but then you'll be looking at going Standalone.

The CBV of the stock turbo vented this back into the turbine housing to help it stop stalling and as well as controlling the amount of air going passed the AMM. No extra air should be going passed the AMM when the CBV recirculates, therefore the ECU knows the amount of air in the system and can work out the fueling according to that.

Ideally, your BOV should be placed after the intercooler as that when the air is most dense.

As it's been mentioned before, ideally, the vented air needs to be recirculated to work with the stock ECU. However, there are members of this forum that have figured out how to 'overcome' this issue on the stock ECU, I'm not one of them!

Hope this clears things up...