help... I'm having trouble figuring out what's wrong with my 91 f150 4.9

Have you checked exhaust system back pressure? The kit is available at most parts stores.

Could be a clogged catalytic converter have system tested with kit tennisshoes mentioned in his answer.

Oh yea, that's another thing I forgot to mention that I did. I cut the cat off too and that didn't do anything.

If the cat melted down then all of that crud winds up in the muffler and plugs it up but not enough to flow some, I would still test the back pressure.

Did you try starting the vehicle with the cat off ? Before you put a new one on.

Yes and that didn't change anything neither.

Ok so ruling out exhaust, then the fuel pressure and flow is next, replace the fuel filter before performing the test so you get a accurate reading, you should also watch for air in the fuel line, that could mean that there is a problem with the pump or connector hose inside the fuel tank. Have you tested for codes in the system? Go to: http://straighttalkautomotive.com/OBD1/ford-OBD-I.html that site has the codes and how to pull them yourself.

Just been thinking about something that happened to me awhile back. Looked at this F 150 forget the year for a guy who had the same problem as yours. I looked at that vehicle for almost two weeks before I had figured out what was going on. Let's just say I replaced a lot of parts on that vehicle before I found out what the problem was. The rubber gas line coming from the metal fuel rail to the connection to the fuel pump supply line on top of tank the rubber hose was collapsing. Dropped the gas tank low enough to see the the rubber line going to tank and watched as the hose started to collapse after vehicle run for about 5 minutes and watched it go back to normal after turning vehicle off . Replace that hose and and the problem was solved. Do not know if that is your problem but I would look into it if I was you.

Alright guys I'll check into that and let tall know how it goes. Thanks

I checked the fuel line and it didn't collapse. I have a 3 inch body lift so I didn't have to drop the tank

Ok so I unplugged the vacuum line going to the fuel pressure regulator while running and it started idling right. But when I plug it back in it bogs down. Bad fuel pressure regulator?

Possible, but it could also mean plugged fuel injectors, what was the fuel pressure.

I don't have a kit to test it I'll have to go get one tomorrow. Also replaced the fuel pressure regulator today