I was wondering if some of you amazing internet mechanics could diagnose a problem I have on my 06 Mark LT with 3v 5.4 l engine.

I just bought the truck and when I test drove it, I made the stupid mistake of falling in love with the truck before really pushing it and testing it.

So the problem is that the truck is very sluggish. When I floor it, it doesn't accelerate very much. It's worse when I'm on the freeway going up steep hills (I live in Colorado). Often I can't keep up my freeway speed. When I floor it, it revs up and stays revved up for longer than usual because it's barely gaining speed. Cylinder 5 often misfires when I do this.

So here's what I have done so far. None of it has helped the sluggish performance or misfire issue:
Changed all 8 spark plugs
Changed all 8 coils
Changed fuel filter
Changed cylinder 5 injector
Ran a can of Sea Foam through it
Always put 87 octane gas in it
*Forgot to say this, but I also had the catalytic converters checked. And there is no back pressure.

Any idea what else it could be???

 

Have the cats checked, may be stopped up.

 

Totally forgot. I had the cats checked too. They're in great shape, no back pressure.

 

Not sure what you're expecting. The 5.4 3V engine only made 300HP, the same as my base 2014 3.7 V6. Yours is coupled to a lousy 4 speed auto while mine has a 6 speed. I've never seen any stock 5.4 3V engine ever dyno anything near 300HP, more like 260 normally. By comparison, the GM 5.7 is MUCH more engine and powerful than the anemic 5.4. Assuming that the tires are still the 265/60/18's, it's not going to fly and it certainly isn't going to press you back in the seat. If the tires are taller/bigger, expect to lose power. It's also going to depend on what altitude you're at as to the power output. The 87 octane gas is absolute minimum for that engine. Might give 91 a try but the difference will be small. A bottle of Techron before an oil change on a half tank of gas wouldn't hurt either. And remember, that truck weighs in at 5400lbs, assuming a 2 wheel drive. Add another 300lbs if 4x4. It's not a lightweight.

 

If the engine is over revving, could the transmission be slipping?

 

My father and brother-in-law both own 07, and 08, F-150s with the same 5.4 L engine, same axle ratio, same everything except for a little bit of chrome I have on the outside of my truck. Both of their trucks run circles around my truck. There's definitely an issue. I've run it at high altitudes in Colorado, and lower altitudes in Nevada. I ran an OBD2 scanner on it while driving and I've never seen it top 100 HP. I also have stock tires, stock everything. It literally runs like a 4 cylinder engine. Either way, I don't think any of what you just said explains the misfiring.

 

I'm no mechanic, so I don't really know what that would sound and look like, but it seems to shift flawlessly.

 

Fuel pressure may be low. Get a dealer shop to check it for you and run some diagnostics.

 

100HP, that's gotta hurt. That YM had issues with the throttle body. You may not be getting WOT. I would suggest taking the intake hose off of the throttle body and take it for a drive. Once at full running temp, stand on it. It should give you LOTS of intake noise, like the old big carbs did. If you're getting puny sound from it, most likely the throttle body is bad....and it ain't a cheap date. That engine is a drive by wire too. You can't floor it sitting still in neutral or just sitting and get the throttle body to go wide open. The PCM cuts it off. The engine must be under load for it tell the throttle body to go for it. If you think the throttle body is the issue, because it's an expensive part, I'd have the dealership check the PCM as it can be defective as well by not telling the throttle body to go WOT. I assume no codes other than the misfire on 5.

 

An idea if someone messed with timing components?

Phasers can be stuck in 0 position causing power loss, or even worse stuck in retard cam timing and it would be a huge loss in power.

Or timing in general could be off, but you usually will set rich/lean fuel trim codes

 

I'm hoping it's something like that and not a compression issue.

 

I bought the truck from a dealer. But I found the previous owner's info and called him. He said he used to have a chip on it. It's no longer there, but could that be affecting the timing still? How would I adjust the timing if it is off?

 

Correct, no codes other than #5 cylinder. I'll try out what you said and let you know how it goes.

 

Don't remember the YMs for sure but some of the 5.4s had bad intake silencers that would collapse. The runner from the air filter to the throttle body has a secondary tube inside. The idea is to isolate the air roar when the engine is under load. The collapsed air tube is cutting the air off and the engine management system is going to maintain the air/fuel so it's like putting a governor on it with weak throttle response. AFIK, there are no codes that go with it so it won't code up.

 

A really dirty air filter could cause it too. When I bought my 02 in 09, the couple I bought it from was living down a short dirt road and the air filter was the original and was really dirty to the point it was restricting air flow and the engine was down on power. They supposedly were changing the oil often? A new air filter boosted the hp to where is should have been and a Superchips program boosted it a significant amount more.