Very few had and still have the separate regulator, anyway if there is a problem it will be with the fuel filter not the regulator. If you would have checked the fuel pressure under load you could have ruled out the pump and filter but not a bad plan to change it if it has never been done.

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