Ok so I'm getting a 2001 f150 with a 5.4 from my dad he parked it cause he couldn't go over 35mph for some reason. He then got a 2008 f150 with the same motor and the same thing happened it wouldn't go over 35 mph. He found out the cat was clogged he disconnected the O2 sensor and drove it and problem went away, so then he took off both cats gutted them out then welded them back on and the problem was resolved. Now the question is could it be the cats are clogged on the 2001 or am I looking at something else?also before he parked it a mechanic ripped him off and charged him $200 and told him the motor was shot he don't even know if the mechanic put a obd2 scanner on it to find out what the issue is. so with him having the same problem with his current truck and resolving it we are thinking it's the same issue.

 

Myself, I would undo the exhaust at the manifolds and wherever else to pull the system back enough to look and see if the honeycomb material in the cat is all melted. Are you doing the work yourself? To be the owner of 2 vehicles with the same problem makes me question if the proper fuel is being used or maybe not doing maintenance as soon as the check engine light comes On.
If the check engine light is On now, you must read the codes as the pcm may have the engine in Limp mode which will make the vehicle feel like it has no power.

 

Yes he maintenance them and they also both have over 200,000 miles he never really had any check engine light on I was just asking because I've been told from numerous people one way to check if your cats are clogged is to just disconnect the O2 sensor and see how it drives because that's what he did with his 08 and that's all it was

 

that will not really work.. There could be some OTHER problem that the O2 is sending a bad signal and disconnecting it masks the problem, but if the CAT is PLUGGED and wont pass exhaust thru it, pulling off the O2 sensor will do nothing... Disconnect the exhaust pipe in front of the CAT and do a test run if you want to prove the CAT is plugged.

 

If the truck really runs that bad........ there should be some CODE that would lead you in the right direction.

 

If you turn the ignition key to Run only does the Service Engine Soon icon light to prove it works?

 

Disconnecting an O2 sensor tells me "disconnecting the electrical connector" which won't do any good for this diagnosis. You can get SOME relief by removing the O2 sensor(s) that are in front of the cat(s), but it won't be much. It is a bunch easier to do than disconnecting the exhaust pipe from the manifold, so a good interim step. With a clogged cat and the front O2 sensor(s) removed I'd expect you'd get your 35MPH vehicle up to 45MPH or thereabouts.