Hey everyone this is my first post here. Ive been pulling my hair out about this. I have a 2004 f150 with the 5.4 2x4. Ive had it for. month now and ever since I bought it it has been giving me both banks lean.(P0174 and P0171) The symptoms I've been experiencing are a little rough idle, but not unbearable. It's a slight shake in park but in drive it's perfect. Before I started replacing parts it would periodically stall when idling after 5 minutes of sitting, Now it will sometimes almost stall but catch itself after the parts Ive replaced mentioned below. Now, if I floor it from a stop, it will get up to speed mediocrely, buck and misfire, and then around 3.5k it will almost catch and take off. Same thing at highway speeds from 50- 70 in overdrive at 1.5k rpm. turning off overdrive definitely helps. The CEL light will flash for 30 seconds or so after it's been bucking for a little. Scanner has show cylinder 7 and 3 misfire, with a random misfire as well. I can take this info and rule out ignition because it only has 70k miles. Ive noticed in the morning when it's cold I have no problems with it bucking around 50. I am a mechanic for a Powersports dealer so I can pretty familiar with engines and I own a jeep as well. So nothing stumps me too hard. Anyways, when I bought it the rear PCV hose was frayed and deteriorated. Replaced that and I was hoping that it would fix the problem, no go. Im not trying to shoot the parts cannon but I've started to replace the main sensors for preventative maintenance. Parts I've replaced: Dorman FPDM, Fuel filter, and Hitachi MAF sensor. Fuel filter was really bad when it was off. Ive checked for any Vacuum leaks with carb cleaner. With my small scanner, I can see that o2 sensors and MAF are in spec. Fuel pressure fluctuated from 220 KPA all the way to 350 KPA. It does loose pressure down to 0 from sitting over night but it will start right up no problem regardless if cold or hot. The guy I bought it from let it sit for a few years, starting it every few months. In a nutshell, im trying to figure out if this is a fuel starvation issue or a mysterious vacuum leak. As Ive mentioned above, the truck was sitting for many years without driving far. Any help would be appreciated as I don't want to pay for someone in my industry to diagnose this. Thanks.

 

Go get the freeze frame data. You have to KNOW the conditions under which the DTCs are set, otherwise you're just guessing in the dark

That code combo is among the most discussed on this and similar Ford forums. A wealth of discussion is already available.

 

Trust me ive combed through every form I can find and nothing is working for me

 

"Lean" codes result from being either "over aired" or "under-fueled' as measured by the O2 sensors. The first scenario can include air not being measured by a correctly operating MAF sensor (vacuum leaks, for example) or a MAF sensor that is mis-reporting the airflow (faulty MAF). The second can include a restricted or diminished fuel supply when the engine demands more fuel. Under acceleration, for example.

Monitor your fuel pressure under the same operating conditions as were indicated by the FF data to see if it's dropping under heavy engine load.

"Lean" codes under load often suggest an inadequate fuel supply. A "weak" fuel pump or restriction in the fuel delivery system are common causes.

 

Im reading pressure on my scanner, which is read by the Fuel pressure sensor. It fluctuates from 30 psi to 40 psi. I just replaced FPDM. Should I look more into the fuel pressure sensor on the fuel rail? The vacuum line that goes onto it has good vacuum.

 

Where is the rest of the freeze frame data? Was there another "page" that needs to be accessed?

FWIW, fuel pressure should be 40 psi as measured by the FRP sensor (KOER).

 

That's the only freeze Frame data I have. I don't have the most advanced scanner. According to the live data, my fuel pressure with bounce around 30-42 PSI under load AND idle. Should I look into the fuel pressure sensor?

 

This is an incorrect assumption. You absolutely need to look at the ignition system first. When these plugs wear and the gap increases the spark will take the path of least resistance and go on the outside of the plug to ground. This causes carbon tracks on the plug and boot. Misfires can contribute to lean codes because the unburned oxygen is read by the O2 sensor. Also, this gen truck has sensitive catalytic converters that will meltdown with misfires.

 

Yeah it's not difficult to pull 3 and 7 and see if you learn anything. Water in bores? Carbon tracking? Condition of plugs?

If there's the slightest chance fake plugs were installed you may find missing electrodes or even missing ground straps.

I'm not saying this is going to create an epiphany, but it's very little time wasted if nothing is found.

 

Okay thank you. I just believe the chances of only a few plugs misfiring is slim. Would that contribute to the misfire ONLY under pretty heavy load? Im going to go ahead and order 8 new Motocraft plugs boots and springs for all the cylinders so I can rule that out completely. If anyone has anything else let me know. Im dreading taking these plugs out.

 

Does the truck only have 70k, or only 70k on the plugs? If they were replaced 70k ago, they should come out ok. I always get her hot then rip 'em out with an impact. Everyone has their preference, YMMV

A cylinder or two misfiring will cause fish biting at low RPM, heavy(ier) throttle, like trying to pull a hill at highway speed without downshifting. Once it downshifts the higher RPM tends to mask the misfires

 

These are exactly my symptoms. Heavier throttle at low rpm and then better at higher. The truck has 70k total. I’m going to assume the plugs have never been replaced. I can confirm this based on maintenance records and suspension bushing condition. I’ve heard multiple ways of removing plugs. I’ve heard more times than not to NOT use an impact and some say it’s the only way. Should I be leaning to ignition now?

 

Move some coils around and see if the misfires follow.

Don't use an impact cold. But again, everyone has their method

 

Why would the truck run and operate fine but as SOON as it get to operating temp, it clicks into the misfire?

 

Could be open loop vs closed loop. Richer mixture during open loop is easier to fire. I always change the plugs with the engine hot and use an impact to run them out. Goes against every rule of thumb for spark plugs but these plugs are different animals. My theory is getting the engine hot expands the cylinder head away from the plug slightly and running them out fast pulverizes the carbon deposits between the plug and head.