Hey there,
I have a 2005 Ranger with a 2.3L. I’ve been trying to diagnose this check engine light for nearly a year now. The only code that occurs is P0171 -system too lean-bank 1. Checked fuel trims. Short term fuel trims are at +20 and increase under load. Long term stay at +25. There is also this ticking noise that occurs under load (sounds like pre-ignition to me). At this point I have no clue what could cause this, it seems like I've tried everything.

Heres what I’ve done:

• Checked for vacuum leaks, smoke tested intake system. Found no leaks.
• Checked fuel system. Found fuel pressure low. Replaced fuel pump, no change. Replaced all fuel injectors, no change. Replaced fuel filter, no change.
• Checked for possibility of the code being a false lean. It seems the truck is actually running lean because when I disconnect the battery and reset the PCM, the truck runs horrible and idles erratically until the PCM commands more fuel. However I have replaced both O2 sensors, Maf sensor (replaced with duralast Maf), EGR valve, and the PCM (replaced with a flagship one PCM).
• I checked for exhaust leaks by smoke testing the exhaust, I found a small leak between the manifold and the catalytic converter, I tightened the flange and found no more leaks. However there was no change in the fuel trims.
• I have found that the engine has some blow-by. From what I’ve read it doesn’t seem like this could cause a lean condition unless an exhaust valve is stuck open, which seems unlikely to me.
• Also the rear main seal is leaking (haven’t wanted to pull that trans to fix it). I’ve heard of cars having a vacuum leak through the crankcase from the rear main, however I plugged all the PCV ports in the intake and found no change in the fuel trims.

At this point I’ve spent way too much money on this truck, but I’m stubborn and would hate for all my effort to go to waste. Let me know if you have any ideas. Thanks.

 

Welcome to the forum

Good testing

You don't mention actual fuel pressure, 60psi is expected by the computer on 1998 and up Rangers, but shouldn't see "lean" code unless it was 45psi or less
There is no Fuel Pressure sensor on Rangers, 2005 computer is just programmed to expect 60psi flow from each injector when its opened

What is the average voltage of Downstream O2 sensor, expected is 0.75-0.85v low oxygen if Cats are working

O2s make their own voltage in the presents of oxygen, they can't see fuel, so the lean and rich thing is a bit misleading since we tend to thing of those in fuel terms
0.1v is high oxygen
0.9v is low oxygen

Computer tries to keep upstream O2 average at 0.45v by varying the Open Time of the injectors
Open Time of injectors is STFT(short tern fuel trim)
Computer calculates STFT 0% based on RPMs and sensor inputs in real time, after engine has warmed up, O2s need to be above 600degF to work

After opening the injectors the calculated time, 0% STFT, computer watches upstream O2 voltage
If it's 0.1v then it changes STFT to +2%, then +4%, then +6%, ect...........until it can get upstream O2 voltage closer to 0.4v average
If O2 is not responding then there are other codes like O2 stuck at low voltage and not responding

The % sign is a real %, +20% means computers 0% STFT calculation was 20% under required fuel needed, so its opening injectors 20% longer to compensate, based on upstream O2 sensor feedback
This is why fuel pressure matters, also vacuum leaks and MAF sensor, because computer bases 0% STFT in this data

Downstream O2 sensor monitors Cat converter efficiency, which effects STFT and LTFT
Cat converters rely on unburned fuel in the exhaust to heat them up and then keep them heated up to burn up toxic gases in the exhaust
If the Cats are working they also burn up most of the remaining oxygen in the exhaust
Which is why downstream O2 voltage is important
0.75-0.85v means most of the oxygen is gone, so Cat is working
If O2 voltage is lower then computer increases injector open time to feed more fuel to the Cats to get them hotter and working
So if Cats are not working then higher fuel trims

Also if Cats or muffler is partial blocked it will cause higher fuel trims as oxygen levels in the exhaust stay high, but this would also cause limited RPMs at higher speeds, as back pressure builds up in the system

And then there is the Computer itself, while rare they can "go insane"
Try a "sanity" test
Clear all codes first
Unplug MAF sensor
Turn on the key and start the engine
Should get a CEL(check engine light) pretty quickly, with "no MAF" code
Plug it back in
Restart, code should self clear
Unplug TPS(throttle sensor)
Same test
You can do this with any sensor, ECT, IAT, DPFE, O2, ect..............

2005 Ranger computer does not have PATS but swapping out computers is still a pain because it does have a Digital Cluster(HEC) which is Paired(married) to the Computer, it prevents swapping in Lower Mile Clusters for re-sale
I think Forscan, a software for Ford Computers, can "re-marry" HEC and new 2005 computer, you can look at the Forscan Forums for info on that

If you are near a City that has "Speed shops" and/or Dyno test shops they may do or known of local programmers than can re-load your 2.3l software and/or "tune it up"

 

Have you checked the fuel pressure regulator?. Is the manifold pressure feedback properly connected? If not it can do as yours is doing and lean out under load. Applying vacuum should lower pressure. (about 4 or 5 PSI difference)