If you have a 2009 or newer Ford vehicle, you most likely have the "Easy-Fuel" system. Ford decided to go to this "capless" system to make things easier for us. Another "Better Idea".
If you have a 2009 - 2013 Ford vehicle, you will have problems with the "Easy-Fuel" system. It may be as simple as having the check engine light come on or you may have your engine stall on the freeway. You can find lots of horror stories on the internet.
The check engine light came on in our 2010 Ford Escape XLT. I went to Autozone and had them read the code P0456 - Evaporative Emission System - Small Leak Detected (FREE at Autozone - $200 at the dealer). If you live in an area that requires a smog check, your car will fail the test until that item is corrected.
I went to a reputable auto repair shop in town and had them do a "smoke test" to locate the leak. It was the flapper in the "Easy-Fuel" system. The Ford recommended fix is to replace the filler neck unit with the new improved version (installed in Ford vehicles starting in 2013) at a cost of about $500 (more at the dealership). I was about to have this done when I found a you tube video describing how to maintain the filler neck so this won't happen again. You can watch it here: https://www.youtube.com/watch?v=JlK2qekqX5s . It shows the problem on a Focus.
Remember that line from "Bonnie and Clyde"? "Dirt, dirt in the fuel line blow'd it away". Dirt and crud prevents the flapper from sealing in the filler tube. The video shows how to keep that flapper seal clean.
Ford knows this is a problem. That's why they redesigned the filler neck. That is also why they had Stant make a locking gas cap for the "Easy-Fuel" capless system. It not only provides some sense of security regarding your tank of fuel, but more importantly it keep the dirt out of the filler neck. You can order this cap at the dealer for $43. I have seen the cap on the internet for as little as $12.42. I'm old fashioned. I like to hold something in my hand before I buy it. My local NAPA store doesn't have one (though the Stant part number 10524 does cross in their stock system). Autozone doesn't have one.
I am off in search of the elusive gas cap. I'll be damned if I'm giving Ford any more of my money.