Here's my theory. The fuel system is closed loop. To comply with epa reg. (Not sure how many of you own old cars but when you overfilled those the vent would spit the gas onto the ground.) Back to the raptor, there has got to be a vent that goes to a part of the intake... And if you give a couple clicks of the handle when filling after it clicks off.. It pushes a little bit of fuel up there and you get a "rich" backfire when it is burned off. If you where watching the afr when the truck backfired you could see this.
Fix- when the nozzle clicks off, that's it, you are full
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