I had the same problem with a cap on a Dodge Dakota. It is one of the safety caps so it can't be over tightened.
What you need to do is pry the outer cap off, then use a pair of pliers to twist loose the inner cap.
I can't get the fuel cap off of my 1990 Escort. It will twist and make the
> usual click, but it won't come off. What can I do?
>
> Thanks.
>
> Bill