Costco has had gas quality issues here. We had a string of customers come into our shop with failed fuel pumps. Some of them customers on their 2nd or 3rd pump. Finally polled them all to figure out what was going on, Costco was the common denominator. I don't remember the specifics... but a sample was sent out and the gas was found to have some flaw that gummed up or burnt out the pumps.

To their credit, Costco paid for all the repairs after all was said and done. And after that incident over a few months we didn't see a customer return. Whether that was because they stopped using Costco gas (shop owner strongly advised against anything but Shell) or Costco fixed the issue is beyond my knowledge.

But I don't think gas is the issue, I'm more inclined to lean towards Amazon plugs being less than 100% quality as suggested above.

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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
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)