Not done this myself, as mine (N-62) doesn't have the disa. I believe you'll need some sealant to re-install after removal, if the seal is compromised during this process (allow sufficient time for this to cure too). Seems simple enough - just one electrical connection and two bolts, gently pry off, inspect flap for integrity, inspect vacuum port is not blocked and all is moving freely. I would give this a shot first, then go further if needed. This is a common fault so I believe and certainly seems to fit the description of your problem. Do a search on this site for more info, as I'm sure many others would have dealt with this.

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08/06 build 4.4i Stirling Grey over black