Having some issues with a persistent P0171 code.

I replaced the fuel filter and cleaned the MAF sensor and had the code come up for the first time about 300 miles later. The truck was running fine and had no issues so I reset the PCM (which I should have done following the sensor cleaning and filter change anyway). Code did not come back after the reset and the truck was running great.

Last week I cleaned the TB due to a sticking blade and performed an oil change. Filled up the truck with gas yesterday and had the P0171 code come back. Once again the truck is running fine so I reset the PCM again to see if the code would come back. It hasn't yet but I haven't driven it much.

I really have no idea what could be causing this code. I can find no vac leaks and I have no starting, idling or running issues. The only thing I have been able to come up with is potentially bad gas as the only two times I have filled up from this one station, I have gotten a code.

Any help/suggestions would be appreciated. Thanks!

 

Different motors have different cures. Which motor do you have?

 

Sorry about that, it is a 2001 XLT with the 5.4 and 4WD.

 

If you TRULY don't have a vac or air intake leak, then you have a upper intake leak, -bank one.

Post your EXACT warm idle rpms in park,- as that can verify. You have to be exact.

 

Warm truck idles at a hair above 700 RPM in park. That's about as exact as I can be, it's just high enough above 700 RPM for me to see the white line indicating 700.

What would cause an upper intake leak? The original upper intake manifold was cracked and leaking coolant so I replaced it about 15k miles ago with a used manifold off a newer Super Duty and I have not had any issues since.

 

That's good, -your right, no vac leaks. Not sure, that DTC shouldn't be coming up.

 

Yea, anywhere between 700-750 rpm is considered to be good. 735 rpms is dead on via program. Your vac/PCV system is definitely tight. If anything, your air filter may be a little dirty at those rpms, but not enough to worry about.

I'd have to agree with bad gas as a possibility. Your engine health is most likely great.

Nope, I wouldn't touch the intake gaskets or worry about it at this point. It's not leaking.

 

If only the one side, P0171, take a look see at the exhaust around the O2 sensor. A leak in that area can cause that code also.

Just a thought.

 

Yea, that is a common problem for model years 2000-2002 5.4L's. Upper intake manifolds are defective and eventually crack. There's no fix other than replacement. F150 specific w/5four engine used UIM replacements could come from an earlier 99 model year which were solid aluminum. The 99's had the ONLY aluminum PI intake made for the 5four and are bullet proof. However, in 2003 Ford upgraded the composite for the upper intakes which strengthened the previous defective area. So replacement choices used were either from the 99 or 03. New, most would go with a Dorman replacement since Dorman had made that affordable. Dorman intakes seem to be a good substitute as there hasn't been any bad reports using them.

Using a Super Duty intake is only possible if the intake came off a Super Duty from model year 2001 +, as Ford used non-PI engines in their SD's. Otherwise, you running a non pi intake on a PI engine. That's not good as it cuts into power and torque. I hope you didn't do that lol. If you did, you have one hell of a restriction for that engine, yea, not good.

In other words, the F150's were PI'ed (power improved) in 99. The F250's were not PI'ed until 01. So as long as your replacement came from model years specified, it'll fit. However, your still stuck with a defective part unless it came from an 03 SD. So keep an eye on it, those will crack eventually as well.

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Other than all that lol. 5fours are notorious for smoking the intake gaskets at the front water jackets. That's where they tend to go bad. IMO, that's not that big of a deal since it takes quite awhile for this to happen. I guess the worse in that is the aluminum heads can pit around the water jackets at the same time. However, that can easily be repaired when it comes time to RR the intake gaskets.

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So, what year SD intake did you use ? Because using the wrong model year from the SD line would create a restriction that COULD easily forward another explanation as to what may be going on with engine. Like, -Idle drop and running lean.

 

Glad to hear it's not a vac leak. It's interesting you were able to point out the dirty air filter, because it is about time for it to be cleaned.

The intake manifold I am running is off an 03 Super Duty so I'm okay there and I replaced the manifold gaskets at the same time.

I do have exhaust leaks on both sides of the motor near the manifold, would that cause the code or does it have to be further down towards the O2 sensor?

 

You said you cleaned your TB, did you remove it to clean it? Or just spray and pray with cleaner..

 

Yes, that can lean it out. They must be pretty bad leaks. Turns the plugs a bright shade of white as well.

Yea, they don't run very well w/exhaust leaks @ the manifolds/thru the Y. So why is it leaking? Is the Trans mount shot? Is the Y set up right? These things are important, - they'll run like a slug as leaks cut directly into the torque and power. Specially low end torque, -she'll dog lol.

 

Huh, -does that really work ? So what, -you just shoot it, then as for forgiveness

 

The exhaust leaks are due to broken manifold studs on either side. Just haven't had the time or the courage to try and tackle them, but if they're causing a lean condition and forcing the PCM to dump more fuel then maybe I should give it a try.

I did not remove the TB to clean it. The area around the blade was pretty gunked up and causing the throttle to stick upon acceleration so I just sprayed that area and wiped it clean with paper towels. Did I damage anything by doing this?

 

Yea, same sort of scenario when running LT headers and HF converters. Has to do with flow/scavenging. With that particular header install, you'll run a little lean w/the original factory tuning, but not dangerously lean.
In your case, with it leaking like that, yea, you change flow, add a hell of a lot of turbulence and bank one + two can't balance correctly. So your AF's aren't going to be even close to what they should be. Perhaps if you had perfectly identical leaks on both sides, -it might lol.
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Yea, you must have a 4x4 for those studs to be like that. So, you have a bad Trans mount then. That's the reason for broken studs. TM's are cheap, easy to replace. Studs aren't that bad to R&R either, -IF you use the right tools. How do you plan to go about repairing the studs?