Thanks, I found an aftermarket inline fuel check valve (8mm) on ebay that I will install downstream of the pump in the supply line and see if that clears up the problem. Since the pump seems otherwise to be fine I think this will help pinpoint it as the source if it clears it up.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz