I've had three Z3's that all had that issue. One car had a replacement done under warranty and another had two replaced. The issue returned each time. I've not heard of an actual solution. :dunno:
However, I've found that holding the belt about a half-inch out from the housing where the belt goes into the retractor and pushing the belt into the housing releases it (sometimes). It is a little like pushing a rope. Also, (this may be obvious) the belt will lock during acceleration and hard turning.
BTW, my current 2002 3.0i hardly ever does it. Perhaps the later retractors were redesigned or modified. BMW could check that for you.
I have the same issue with my car. My wife is pretty much the only person who ever gets in the passenger seat, so I cheat by putting a clip on the belt to keep it from retracting too far. Works great, but only for her, else I have to adjust the clip.
You could search for other threads where this has been discussed.....it's a common problem.
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