The flap throttles intake air at low engine load to help create an intake depression draw in the exhaust from the egr valve, it also closes completely for several seconds when you turn the ignition off to give smooth run down or 'anti shudder' it is also a safety shut off device in case of engine running away. I think youll find there is no vac pipe attached. If your egr is blanked off at the vac hose but not deleted from the ecu, this flap will possibly be trying to control non existant exhaust gas supply.it will not be closed completely when running but when you suddenly reconnected the egr it could have been trying to control it and choked the engine, hence white smoke , These flaps are also prone to fouling up and the plastic gears strip.