Although I had this happen in my 1998 Ranger XLT, I just had one of those new safety gas can spouts get jammed in the filler neck and it would not come out. I was emptying some gas to rotate the fuel for my generator into my Ranger when it got stuck and when it did come out it pranged the hell out of the flapper valve. It actually ripped the **** thing out and it was jammed in the hole. I had to poke it back though the hole!

 

I hate those new spouts. I ordered two of these for my gas cans:

https://gasspouts.com

 

I'm sticking with my old plastic jugs just because of those new stupid spouts!

 

I believe our Mustangs come with a funnel that we are supposed to use when filling with a gas can. It should be in the spare tire area.