incrediboy729

I have been battling getting a P0171 lean code in my 99 F-150 5.4L for the past few months. Here's where it began:

First Mechanic Diagnosis:

Check engine light comes on while driving, I pulled code and behold, P0171 (Lean Bank One). Take it to my local (and trusted) mechanic. He finds a vacuum leak, replaces the tube, resets my computer and sends me on my way.

Few days later, light comes back on, pull the code, P0171. I test the vacuum lines myself, no leaks. I also clean my MAF, test my PCV valve and reset the computer. Light comes on next day.

Second Mechanic Diagnosis:

Finds very small leak coming from idler control valve, also a possible lazy O2 sensor. Resets fuel trim and dumps in some injector-cleaner in hopes of the computer being able to compensate for lean condition, since he knows i'm a poor college student. Tells me to come back if it comes on again.
Light comes back a few days later. I replace both upstream and downstream bank one O2 sensors myself. Reset computer. Few days later, both a P0171 and P0174 code now appear. Lean on both banks.

Third Mechanic Diagnosis:

Take back to mechanic. Idler Control Valve is replaced, MAF is replaced. Fuel pressure is checked.
Drive around for another few weeks, about 2 weeks later, P0171 is back.

I'm about at my wits end. I'm a poor engineering student looking at graduate school and I don't have the time/money to keep dealing with this. Can anyone think of anything that could cause this?

Additional Detail: I do have a AEM CIA installed. However, I do not believe this to be the problem as I've had it installed for years with no issues.

TL;DR:
Truck running lean, MAF, Idler Control Valve, and 02 sensors replaced. No vacuum leaks, fuel pressure is good and PCV is good. Stumped at what to do next.

Any and all suggestions are MUCH appreciated. Thank you so much!

DaughtryLc

I had a K&N CIA installed that killed my MAS air....yes..I may have over oiled it...took it off for good after that...

Anyway, it was throwing the same code but you say you have replaced the MAS...

I would say remove the CIA and just try it without....it want cost you nothing to try..and you will have ruled that out also

incrediboy729

Thanks for the suggestion!

I would try that, except the original can't be put back on without buying a few parts.

Like you said though, the MAF was replaced so I doubt it's causing fouling; it's also an oil-free filter.

A7X

When's the last time you changed your fuel filter?

A lean conditions could also be a fuel delivery issue...

Also, check the O2 harnesses... if they have the slightest bit of corrosion you'll get a slow response or bad reading...

Do these things and come on back...

Ringleader

Could be leading intake gaskets.

Mudder5

Ive been fighting p0174 the bank two running lean haha, i have done intake gaskets and vaccum hoses, still havnt fixed it.

A7X

I was throwing a lean code on bank 2 for a while...

Sprayed all around the intake, cleaned maf, checked hoses, replaced a couple, replaced fuel filter...

Still there...

I was finally gonna try to replace the O2 that was throwing the code, unplugged it and it had the green corrosion powder in the harness... cleaned, dielectric grease and problem solved... to this day (4yrs later)

Mudder5

Ill look into that

Frank_Ford

You seem to have done all the right things and a few extra things (like changing out the MAF: usually just cleaning it does the job).

You said that you checked the PCV valve and it was OK. These only cost about $10 and are easy to change so why didn't you change it? I would change it and change the vacuum line to it just to make sure. It is good maintenance and real low cost and effort.

The other problem (and more likely the cause) is most likely a air leak somewhere between your air intake inlet and the engine. This could very well be the aftermarket air filter you installed. I would set everything back to stock like it was when they built it at the ford plant and see if that fixes your problem. also look for an air leak anywhere in you intake system / manifold.

Good luck.

Mudder5

My pcv valve has the molded hoses and stuff, it was $80