2005 ranger with unknown year engine, I believe its from an 03 but guy we bought it from screwed so much up not sure we can believe the year either. Issue started about 8 months after purchase so I don't think it's related to that anyways.

Sometimes the code is bank 1, sometimes bank 2 currently it's both. There has been times the codes have been gone completely.

I have been chasing this for a few months, along with multiple trips to 2 different mechanics with no luck. MAF, IAC, TPS, O2's and ECT have all been replaced. Has been smoke tested twice with no vacuum leaks found. Fuel pressure tests ok at 65 PSI while running. Mechanic tells me fuel trims are high at idle but within limits while driving. Also noted that while driving when you lift off the throttle they spike up again, so issue seems to only be while not on the throttle. Have checked air intake lines and no obvious cracks found, doesn't sound like there are any exhaust leaks.
Last I checked, if anything the plugs look like it's a little rich, which doesn't make much sense given the lean code.

Any suggestions as to where to look next?

 

I'm having the same issue... same codes... I tried the same things... and still the issue persists. The only thing I can think of that I did not try, is changing the 3rd (downstream) o2 sensor. I will be changing that today (if the part is in stock). I have a 1999 3.0. If it fixes the issue, I will let you know.

 

im not sure how to check this, you should be able to get info here on the forum. check the fuel pressure regulator I think its on the fuel rail with a vacuum hose to it. pull off the hose and see, smell or taste for gas. if so its leaking. this could do it I think. the 3rd down stream O2 sensor is just for smog test not tuning the motor.

 

The below thread has photos showing the location of the fuel pressure dampener. Only a few years had them, and not all 3.0's will as I've seen some without.

OP, if your truck has this, pull the vaccum line off. If you smell fuel or you see fuel coming out of this, the dampener has failed and needs to be replaced. The extra unaccounted fuel going into the engine would cause the fuel trims to go into the negatives as the computer is subtracting time from the injectors due to the unaccounted-for fuel. It also may result in hard starts as the engine may flood with extra fuel going in via the vaccum system.

('99 V6 3.0) Fuel Injector Damper on Fuel Rail Leads to ... ? - Ranger-Forums - The Ultimate Ford Ranger Resource

 

It does not have a damper

 

I believe it's a fuel problem, it's on both banks so not an injector? some thing is making it get too much fuel, thats what I think. keep looking.

 

For the record, I had P0171/P0174 codes on my 2003 3.0 about two or three years ago now. Truck ran fine, just had wacky fuel trims. Fuel pressure was fine as far as I could tell on my gauge, but I decided to just replace the fuel pump since it was the original and had 200,000 miles on it. That fixed the issue and the codes went away.

 

mechanic seems to think it's too much air and the fuel supply is ok. But really only so many places extra unmetered air can get in and nothing can be found.

Looking at the plugs, if anything they look like it's rich not lean, so not sure why that would be

 

I have considered doing that too, but it's about $400 for a replacement pump assembly, that's a bit much to just throw at it hoping for the best. Was your pressure bleeding off at all or holding steady all the time?

 

I just wanted to report back: Changing the downstream o2 sensor did not help but frankly I did not expect it to. I only tried it because I was running out of options..............

 

Holding steady regardless of where I held the throttle, which made it hard to diagnose.