I have a 1998 mustang. Can floor the fas pedal and I won't accelerate in fact I decellerate and u can hear it sputtering out my exaust. And most of the time I can't go over 20 mph plz help.

Possible plugged catalytic converter?

Well I had my mechanic take a look tht was his first guess but then he listened to it and then he said it could be the fuel filter or pump but he can't tell.

Easy enough, Have him change the fuel filter, and check fuel pump pressure. For a Ford, I don' know what it should be, you can look it up on the web... Bad fuel pump can cause these symptoms....

If the converter is clogged, you won't get much exhaust from the tail pipe.

It ran perfectly before these symptoms occurred and the CEL wasn't on for the longest time and it would do this until yesterday when I was going up a small hill

I also thought it was the throttle position sensor but its good

Did you end up finding out what it was?