Oh, since the servotronic valve is supposed to be normally open for bypass (minimum assist), could I try applying a fixed voltage to close it (maximum assist) to see if there's any difference (basically just to rule out the pump itself)?

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2008 X3 (E83-LCI) 3.0si M-Sport
2004 X5 (E53-LCI) 4.4i M-Sport
1994 318i (E30-LCI) race car - sold