Maybe take the right hand sender out and work it up and down while watching test 6 and the fuel gauge. It sounds like you have a problem with the right hand sending unit, The float can easily get hung up in the small space in the sump on the right side. Make sure your 2 syphon jet lines and the relief valve line are clipped into place.

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