idle is simply set by a screw on the throttle body, the one the rotating throttle piece (cable is attached to) hits when it return to the normal position, if you want to change idle then just screw it up or down to achieve what you want

if the smoke is coming out the dump pipe then its either leaking seal in turbo, leak in cylinders in motor or if like me recently there is oil coming from the motor into the inlet in front of the turbo because of dodgey PCV valve/blocked air filter/no catch can

BOV could not affect any of this, no exhaust is certainly gonna make it run poorly and affect idle