I figured out why I'm getting 'check engine' lights. My gas filler door, when I'm on the freeway, is pulled open only slightly by the negative pressure along the side of the truck. Consequently triggering a light.
This isn't just a F150 design, but a Ford design. My wife's Flex has the same filler door.
Anyway, is there a way to increase the tension of the spring holding the door down? Or do I need to replace the door?

Thanks.....