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The valve is part of the BMW Active Exhaust and is purely for noise reduction.
It is operated by a vacuum pump underneath the trunk (a common mod on the E90s is
to disconnect the pump via the electric connector in the trunk to keep the valve open).
The basic system was initially introduced on the E46 platform and the electrical function
of the exhaust flap operation is controlled by the MS 41.1 module.
Where at idle speeds, up to 2500 RPM: the exhaust flap is closed with vacuum applied to the actuator. This causes the exhaust gas to pass through the long resonator pipe, reducing the low frequency noise that is generated.
On the F10 it is now reverse where vacuum applied means the exhaust flap is opening
There is no button/setting which will keep the valve permanent open ,the only option is to
"hard wire" the valve in the open position and seal/plug the vacuum line.
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