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Try a little silicone on the catch mechanism. By the way your explaining the issue, it obviously not compressing and setting fully when you close the door.
Been a few threads on it. Mine stopped doing it after a couple of months so I did not pursue it any farther. I do not think anybody on the board here that I know of has yet taken it in for a dealer service visit for the issue to see if there is any input from Ford corporate on it as to cause and why.
Short explanation of the catch mechanism; catch when opened full length it vertical as pictured;
When closed is horizontal as pictured;
Fits in the fuel door vertically when opened and first closing. As you push on the door the catch rotates horizontally in the door slot and locks the door. See door slot as pictured;
First check the slot for any debris of possible plastic molding left on either side of the slot. If there is remove it with a razor knife carefully.
If not, leaving the catch plunger mechanism extended use some silicone lube spray or Vaseline on the exposed portion. Push it in and out to disperse and film the plunger.
Mine is well lubricated, so it might be yours is dry and catching partway on the dodo slot, not fully rotating to horizontal in the door slot.
This doesn’t work, take it to the dealership service.
I had the same problem. As it turned out every time it happened was after putting something in the bed. Apparently (in my case) just leaning on that door , even a little) will cause it to open. It hasn't happened since.
+1
Either I was inadvertently leaning against it when placing/removing objects in the bed, or it just needed to “break in”. Mine hasn’t popped open unexpectedly in about a month.