I seem to remember checking the DISA by manually moving the flap and putting my finger over the vacuum fitting to see if the diaphragm holds. The flap will close part way but should hold till you take your finger off. The solenoid must be normally open if I recall this correctly.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)