I Have a 97 f250 5.4 v8 . It has 325 000k. It runs rough in park at low rpm's and has all it's power but when I put it in any gear and try to drive it there's next to no power at all I can floor it and go very slow, I just replace all new plugs,coils,fuel injectors,intake,all gaskets,a few vacuum lines, throttle position sensor,mass air flow sensor,throttle body, PVC valve and I don't know what else to do, it's completely gutless unless it's in neutral or park. PLEASE HELP ME!

 

I'd suspect clogged cats.

With that many miles on it, I would have done a compression test when the plugs were out.

 

Definitely due for a compression test. Start there..

 

A muffler shop can test the cats and the rest of the exhaust for blockage.

 

Sounds like the fuel pump IMO.

 

Yeah fuel pressure test. Compression test Cats checked for clogged. All that is possible.

 

Right, -but fuel is first..Always IMO.

 

I just noticed - you didn't replace the fuel filter?

 

I have a 97 ford f250 that I had parked over night and the next day ran rough and no horse power at all so I went and replaced the intake gaskets, coils,plugs,vacuum lines,fuel pump, fuel filter,fuel injectors, and now it has power when you step on the gas but it looses it after a few seconds and the comes right back it will not run in the cold it just cranks and back fires, when it does run it's constantly backfiring. What could cause this? That's all that I did to it so far

 

I took the cats off and it ran the same

 

Compression test?

 

no I don't have the equipment to do that, but I shall get it

 

Parts stores should have compression testers in their tool loaner kits. They aren't expensive to buy:

http://www.harborfreight.com/8-piece...kit-69885.html