I have a 97 F-150 4WD, 5.4L. Recently got check engine light and bad miss at idle, sometimes stalls at idle but starts right up. Pulled codes P0171 and P0174. Lean mixture in left and right banks. My research indicates most likely bad MAF or EGR. Is there any way to narrow it down instead of just throwing parts at it? Only seems to be a problem at operating temperture, idles fine when cold and runs great above idle. Also, for what its worth, I replaced the O2 sensors about 6 months ago due to a different code. This problem came on quickly, not gradually. All maintenance items- plugs, fuel filter, etc.. changed less than 20,000 miles ago.

 

The P0174 / P0171 codes are posted so often I'm surprised you didn't come up with the most common answer useing the search function. Odds are your PCV line (rubber boots) are shot causing the lean codes. Ford base #6C324 includes new PCV the tubing and the 2 rubber boots, also check the vacuum line from the VMV next to the brake booster that runs down to the intake (green clip)

 

I saw posts refering to the same codes, but the drivability symptoms were different. The fact that it doesn't happen when cold, but does at normal operating temperture is what has me stumped

 

Also, wouldn't bad boots or a vacuum leak generally come on gradually, not suddenly.

 

The lines degrade to such a point that they're working fine one minute, and when a portion of it goes or springs a leak, you've got your unmetered vacuum leak leaning out your system.

 

I just popped the hood, and all the boots and all vacuum lines appear to be intact. I'll check a little more thoroughly tomorrow, but any other ideas?

 

Had the same two codes, K&N oiled filter, Cleaned the MAF sensor and it has not returned! (3k miles) yours might be more, try this before you start throwing parts at the codes! just a thought.

 

I found a bad boot yesterday going into a valve on the back of the intake manifold. Replaced it and problem went away. Thanks for all the help.