As far as the gauge, maybe check the connector and then take the pump out again and have a look, maybe something obvious with wiring or a bent sender contact. As I think I said before the picture of the Hella pump at RockAuto showed one of the contacts oxidized already so it would be a good idea to have a look at that and use some contact grease if you haven't already.

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