Fuel trim on my 5.4L 3v 2004 F150 seems out of whack. I'm having other driveability issues (mainly a P0301 at extremely high load that doesn't move when I swap COPs or plugs or injectors; compression is good in that cylinder). I'd like to see a more normal looking fuel trim before chasing the misfires more.

I drive at altitudes of 6000-7500 feet. I do not have any intake or engine modifications. Long term fuel trims are >0 across the board, usually >10% for any highway driving. Under heavy load, the long term fuel trims seem to max out at about 18%, and the short term at about 27%. A nasty side effect of this is that I instantly go very lean if the computer goes open loop under heavy load (as STFT goes to zero).

Things I have done: Replace fuel filter, replace fuel pump, clean MAF sensor.

The "BARO" PID seems appropriate for my altitude (about 23 "Hg). The cats seem healthy (downstream 02 sensors don't oscillate with the upstream ones). Fuel pressure looks good on the rail. I don't have any pending DTCs except for that P0301 under very heavy load.

Any suggestions?

Thanks!
fiz

 

If I didn't know better, I could guess you have some whacky gas.

 

Who wrote your tune?

 

I would assume Ford; I bought the truck used about 10 months ago. This issue developed within the last 6 months. How might I determine if it has a custom tune?

I've tried gas from several different stores without any change.

Thanks,
fiz

 

The PCM is supposed to go to open loop under heavy load.

Although first you say short term reads +27 under load, then you say it goes to zero under load when it goes to open loop (which it should be in open loop anyways under heavy load). Which is it?

I can't really remember off the top of my head, but short term may read 0 under heavy load since the PCM is in open loop and not paying attention to the upstream O2s. It's using long term fuel trim to make big adjustments in a hurry so you don't get run over as you're accelerating under heavy load.

The only purpose of short term fuel trims is to make the upstream O2s switch, significant fuel adjustments are done by long term.

 

Both. I start up a steep hill at WOT. It's clearly closed loop for a while because the STFT is nonzero (rising to +27 and staying there). After a few seconds, it goes open loop (STFT goes to zero and the scan tool shows Open Loop). This is coincident with the MIL flashing and a pending P0301 being set. I can't tell if

(1) it misfires because the truck went open loop due to load or
(2) it goes open loop because of the misfire (as well as shutting down that cylinder) when the MIL starts flashing.

Because of the difficulty disambiguating these cases, I decided to focus on the large LT and ST fuel trims first...

 

Hmmm pulling one out of my drawers here, did you check for vacuum leaks>? check for exhaust leaks (close to/at the manifolds)?

 

I've been turning those ideas over, as well as a misfiring cylinder that isn't setting codes at normal load. I imagine that would make the exhaust look lean, which would make the computer jack up the fuel trim.

 

Well, you've got a misfire on only one cylinder. If it was fuel related, I'd be inclined to believe that it would/should affect multiple cylinders.

Checked compression? Leak down? Plugs?

 

Vacuum leaks would show themselves as high long and short term fuel trims at idle, then the trims would return to zero or thereabouts off idle.

 

Me too, but it is the farthest from the fuel inlet...

It held at about 165 psi on both cylinder 1 and cylinder 2. I didn't do a leakdown because the compression looked good.

Motorcrafts about 5k miles ago. COPs are new. I swapped COPs/plugs/and injectors between cylinder 1 & 2 with no change in the code.

 

Just because compression is okay doesn't necessarily mean that leak down will be okay as well.

My gut is telling me that you've got a specific problem with that cylinder, and it ain't fuel related. I've seen a lot of broken valve springs in '04ish 5.4 3Vs. I'm thinking you may have a slightly weak valve spring on that cylinder.

What about engine vacuum? What's it look like at idle?

 

Leakdown is about 5%.

I'm tempted to pull the valve cover and replace it. Can the head be removed in the vehicle?

About 17 psi at idle (dead steady). Drops to 13 psi briefly after acceleration to 2k RPM, then quickly recovers to about 17 psi when holding at 2k.

Thanks,
Ben

 

What device or software are you using that yields a % on the STFTs?

Mine shows stoich at around 1.?? in CL and stays at .820 in OL, SCT software.

 

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