I pulled the plenum today and found the culprit... an over-tightened hose clamp had cut into the short section connecting the fuel rails! Pretty serious leak, no telling for how long!
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While I had it apart, I refreshed the paint on the block that the gas had eroded.
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Done! Running like a champ.
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Please change all your 'auto parts store slotted hose clamps' to BMW solid style hose clamps.