I have searched the forum for months, and I have tried much of what has been posted. I have a 2005 F150 FX4 5.4L with 200K on the odometer. Previous owner stated the motor and transmission was replaced around 50-60K ago. I did not get any proof of the replacements, however the throttle body and exhaust would suggest it was only a block change. I digress, the truck runs and idles smooth, but there is no top end. With the foot down, the truck screams but isn't going anywhere. Seems like it runs well up to 30-40 mph and then there is no more power. It feels like there is only 200hp and not 300hp. I can drive 80mph (Wyoming Interstate) with no issue other than lack of power.

I have not changed all the spark plugs or coils. I had a miss and the dealership identified two cracked plugs (5 and 8 if I recall) and a bad coil on 8. The truck at that time had the same acceleration issue, but also had shudder under load going uphill. The two plugs and a coil solved the shutter issue but not the acceleration. I have not done anything with the injectors. I recently checked the back pressure and there is no restriction in the exhaust. Oil changed every 3k, new fuel filter and air filter every 3k (just to rule it out). Everything changed has been motorcraft.

The only other numbers I have to produce is from my live data that showed fuel pressure at 40 psi, MAF maxed is about 18 lbs/m and downstream bank 2 oxygen sensor does not level out (it continues to fluctuate voltage like the upstream).

I don't mind trying to solve the problem, but I want to start in the right direction. I'm a bit confused as to why it runs well but has no power. It feels like I'm towing a load, only will spin the tires on ice.

 

How warm / hot are the brake rotors & wheels after a bit of a drive? What does the Automatic Transmission fluid look / taste like?

 

Check the air intake in the fenderwell - I've heard of insulation getting sucked into it blocking air.

 

Wyoming is fairly high in elevation which reduces the HP significantly. Those 5.4s aren't that powerful in the first place. I test drove a one yr old SWB 2005 4x4 with the 5.4 3V and a new 2006 scab 4x4 with the 5.4 3V in 2006 thinking of buying one and both were slower than my 2000 Scab 4x4 5.4 2V.

My current 2002 felt slow when I bought it a 69k miles. Then I found out it had the original air filter and the PO had moved to a home on a dirt road resulting in the air filter being the dirtiest I've ever seen. A new air filter did wonders and then a Superchips program really woke it up.

 

New brakes all around less than a year ago. Just checked last week to see if the pads/e-brake were dragging. Also did fluids all around a few months ago. Trans fluid looked good and complete flush was done.

 

I agree with you for the most part. I had a 2004 5.4 a few years ago and don't remember thinking there was a lack of power. I recently ran a 0-60 test and it was 12 seconds.

 

Is there a way to check the timing advance? If the computer isn't advancing the timing, it wouldn't have any power but would run good at low rpm. Maybe a computer reflash by the dealer?

 

I think my scan tool can can read timing...I'll look into it. I asked the dealer to reflash when they diagnosed/replaced the plugs/coil. They told me it was up-to-date and would not do it. I could always ask again.

 

I’m curious as well. Mine does the same exact thing. Engine and transmission rebuilt as well

 

This is a very old thread. But, my first guess would be clogged catalytic converters. Get someone to check the back pressure.

 

My 2010 F150 5.4L has the same issue at 120,000 miles. It drives and runs fine part throttle. If you go WOT the engine rpm will increase slowly like it's being choked of air. The accelerometers in the TorquePro app indicate my 6000lb truck is making around 90HP max. My first thoughts were clogged cats, intake obstruction, bad throttle body, clogged fuel filter sock in the tank. It seems to be getting worse over the last year. I just bought a new bluetooth OBDII adapter (OBDLINK MX+) that reads more pids than the old ELM 327 bluetooth I was using with TorquePro, but I haven't spent much time trying to figure out what is going on. It appears commanded and actual throttle response are good, so I'm going to put an actual fuel pressure gauge on the schrader on the fuel rail next. I can't find any fuel pressure I can read off the OBDII port yet.

EDIT: I'm finding threads with similar 5.4L top end power loss where the fixes end up being fixing broken timing chain guides or new shortblocks due to bearing wear causing loss of oil pressure to the cam phasers. It's all over the place. All the threads on forums indicate no codes thrown through it all.

 

So I datalogged the truck and figured it must be air restriction. I started taking the intake apart and decide to stick my camera in the fender tube from the intake box. A piece of closed cell foam was lodged up in the fender blocking the tube. All the air was coming through the dime sized holes in the tube creating a sucking sound during wide open throttle. Problem solved for me. Now, the question is was this loose foam from the fender or a foreign object that somehow got up in there?