There should be a Schrader valve at the front or on top of the fuel rail.

A weak pump might not really start giving enough fuel pressure at starting voltage, but gets going enough at 13.5+v after running voltage.

Just a theory though, you're right to do diagnostics. I'm the type who enjoys using the parts cannon method of elimination. Especially on original parts.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
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