This might be obvious to many of ya, but it came as a surprise to me this morning. Glad I 'found out' here at the office, where I can prepare for it rather than out there hundreds of miles from anywhere...
If you look at the filler hole, you'll see there's a metal door in the way of the nozzle. You can press on this door with your finger or a pen and it wont open. It needs to be unlocked before it will move. This unlocking action is handled by the nozzle of the pump itself; the outer diameter of the nozzle engages two tabs on either side of the door (shown in the pic below, pointed at by the green arrows) and that simultaneous pressing-back action releases the door, which is then pushed out of the way by the tip of the nozzle.
If you look at our Ford supplied funnel, the neck is modeled after a pump's nozzle so the same unlock-and-push-out-of-the way action occurs when you shove it down in there.
This is all fine and good, but I have fuel cans. And these fuel cans don't have the same diameter nozzle. In the past I've used the Ford funnel in conjunction with the fuel can's nozzle and that seems to work ok (I only did this because I was using the Ford funnel as a funnel, not realizing I was unlocking the door down there also), although you'll loose some fuel and you'll be standing there for 5 minutes waiting on 5 gals to pour.
So this morning I grab myself a full sized funnel (to carry with me in my toolbox, to speed up fueling via cans and to free myself from those cans' nozzels) and I pull out the Ford funnel from the truck and shove it in a drawer never to be seen again.
Thankfully, it occurred to me shortly after that I should probably test my plan before relying on it, and sure enough my funnel is a no go. The knowledge I've just shared above then came after a few minutes of figuring out why.
Here's the solution I came up with:
I cut my funnel's nose off (was a smaller diameter than the fill neck) so that it's starting diameter was slightly larger than the fuel fill's neck. I then cut a slit in the filter, angling it so that it will collapse in on itself as I press it in the neck. After a bit of tweaking, I've got it so that the collapse happens just right to still press on the tabs, unlocking the door and yet still with enough length to push the door out of the way and hold the funnel in place for hands free filling.
Moral of the story? Don't loose the Ford funnel. Or if you do (or want to replace it for reasons like mine), make sure you dial in your replacement funnel before you need it.
Hope that helps.