I’ve been dealing with these lean codes and it’s starting to appear to be fuel related. This is on a 2005 Ford ranger edge with the 3.0 5 speed. The codes I’ve been getting are p0171 and p0174 lean both banks. I have a really weird symptom which is whenever I relieve the fuel pressure and turn the truck back on my fuel trims drop back to normal +\- 10 for about an hour. Then they go back to 15 or 20 and over and I throw the code again. I replaced the fuel filter with no difference. I know the fuel pump was replaced about 4 or 5 years ago amd possibly it’s aftermarket. I checked the fuel pressure amd it’s about 40-45 KOEO and when I turn the engine on I’m reading 50-55. When I turn it off it goes back to about 40 and stays there. I havnt been able to load test it but anyone who can point me in a direction would be huge help as I don’t really know where to go from here other than replacing fuel pump assembly from Ford

 

Fuel pressure looks fine

Lean or Rich from codes is Oxygen related
O2 sensors can only see Oxygen, they use a chemical reaction to generate their own voltage in the presence of oxygen in the exhaust
They must be heated above 600degF for the reaction to take place, which is why O2s are Heated
0.1v is high oxygen
0.9v is low oxygen
So O2s generate less than 1 volt

O2s start to run out of chemicals after 100k miles or 12 years, these are the ONLY sensors that wear out, like tires, brakes and batteries, they just wear out

So if you don't know the history of when O2s were last changed then change them, period
The problem with older O2s is that they start to cost you MONEY in lower MPG over years, which really adds up, and won't set code until they have each cost you $300+ dollars in wasted gasoline

If O2s are KNOWN to be good, for sure
Then Lean on both banks can be dirty MAF sensor, or UPPER intake vacuum leak(but would have higher than normal idle)

At 50-55psi running fuel pressure no lean code would be set, for fuel reasons, on a 1998 and up Ranger

 

Thank you for The super detailed reply that’s exactly what I needed to hear. Bought the truck about 5-6 years ago with 95k miles and today it has about 150k with some change.

I’ve had this lean issue for about a year and it has never once lit the check engine light without both banks being lean so I never suspected the o2’s to be bad because it seems like both banks started running lean at the same time. It’s also never really run super bad the only reason I can tell is it doesn’t quite run right at idle and it has a very slight miss that the computer won’t pick up.

The thing that’s weird to me is I’ve that every time I relieve the pressure in the fuel rail and reconnect it the trucks fuel trims go back to normal for an hour and then lean again. This happened when I did the lower intake manifold gaskets and when I replaced the fuel filter both times I had to relieve fuel pressure and the truck ran in spec fuel trims (+\- 10) for an hour and then back to running lean(20+).

I think I might as well do the o2s since I’m about to go away for college this semester and won’t have a garage to work in

 

Yes, do both upstream O2s for sure, downstream O2 is your call but it does effect LTFT(long term fuel trims)

Computer calculates fuel injector open times based on RPMs, throttle position, and MAF/Air temp data
That's STFT 0, it then ADDS LTFT to that for the new STFT 0

LTFT is there so the system can age gracefully, there WILL BE small air leaks and slightly lower fuel pressure, dirty filters and injectors over time, that's life
So LTFT is saved in memory so computer doesn't have to re-learn the system with every Key On drive cycle

So if LTFT is +10 then STFT 0 is actually +10, so if STFT goes to +10 then you are actually at +20

And at the next cycle of key off and on LTFT may then be at +15, its an average of STFT over the time of this drive cycle

If you change O2s and that was part of the issue you should see say -5 STFT if LTFT was at +15
Over a few drive cycles LTFT would start coming down because of the -5 STFT lowering the average
Then STFT would eventually get to +5 to +8 cruising which is what you want to see, and LTFT under 10


There would be no connection that I can think of between 0 psi fuel pressure at startup(well no start maybe) and lower fuel trims for an hour, or even 5 min
There is no fuel pressure sensor on Rangers
And pressure would be 50-55psi instantly as soon as engine is above 400RPMs, i.e. fuel pump is on full time

O2s need to be warmed up which only takes 5min from cold start, before that computer is using STFT from tables in memory, so not really valid numbers that mean anything
STFT is only valid when cruising, not when accelerating or decelerating must be at a steady RPM. and not at idle, idle is rich on purpose so engine doesn't overheat

Also the Downstream O2 should have a steady voltage, 0.7v to 0.8v if its operating correctly
If its jumping around like upstream voltage or lower voltage replace it, but usually means a Cat converter is starting to fail

 

Both my upstreams bounce between about 0.0-0.7

My bank 2 sensor 2 seams to stay at .6-.7

and my bank 1 sensor 2 seems like it stays at .7 for a couple seconds then dips down to .1 very shortly then back to .7 for a couple seconds and repeats that

 

0.6-0.7 is a bit low so worth replacing it, in my opinion, but can wait until funds permit, its not causing Lean codes
Yes, the computer adds extra fuel every few seconds to "feed the cats", lol, Cat converters need raw fuel to burn up and stay HOT, downstream O2 sees that but otherwise stays steady

Do you have true Dual Exhaust system, or single exhaust with 2 downstream O2s in the Cats?
If single exhaust then the B1S2 is the one closer to the engine so would show that bounce every few seconds vs the B2S2 which farther back behind another Cat Converter

You want to see 0.2 to 0.6 from upstream in steady driving, so may be time to change O2s

2005 is 17 years old now

 

I have the y-pipe with a cat for each bank and an o2 before and after each cat

just set the lean codes driving back from taco shop freeze from showed the fuel trims between long and short term at bank 2 were added up to 25 so it set the code. Set the code for bank one as well. Never sets the code for just one bank

my freeze frame always shows that it trips the code under acceleration never at idle

 

Saw that the upstreams o2 are 100 a pop from Ford yikes, I think it’ll be worth it with saved mph tho

 

All the O2s are the same on all Rangers, 1993 thru 2012

O2 part number differences are the Length of the attached cable, nothing to do with location of the sensor on the exhaust
$100 is ridicules for one O2 sensor

Bosch O2s are factory, they run $25 to $40 each but often sold as set of 4, like these: https://www.ebay.ca/itm/184961260550...EaAgwhEALw_wcB

Set of 2 with short lead wires and 2 with longer leads, $90 for all 4
Make sure they have the round connectors attached, some at a lower price will be Bosch but no connectors so you have to splice wires, PITA

 

I think I’m gonna buy the two upstream for now and see how my fuel trims do

Autozone Has Bosch for 60 a pop I think I’ll try that because they have it in stock I’ll let you know my findings

 

Unhook the battery while changing O2s
Because its just a good idea, but also if battery is unhooked longer than 5 min the computer test all the systems once restarted and warmed up, takes a couple of drive cycles
Heat up above 160degF and cool down under 120degF is 1 drive cycle

 

Same codes are back

 

It seems to run better though

 

So you know its not false Lean

If its showing Lean on both banks then it has to be something shared by both banks

If memory serves you checked fuel pressure, above 50psi and below 70psi, so OK
And unless 2 injectors on each side have limited flow then its not a fuel system issue

That leaves under reported air or air weight
Unreported air is an upper intake vacuum leak
Air weight is MAF sensor or Air Temp sensor(IAT)

Look at live Data for air temp after warm up, in 2005 should b about the same as outside temp
And MAF sensor at warm idle should be, 3.2-3.8grams per second, gr/s or g/sc on a 3.0l

 

It seems to me like it’s not a vacuum leak as the freeze frame is never at idle it’s always 2k+ rpm under load while driving

i do know that I am running a duralast maf that shows 4+ g/s and the iat shows about 100 on cold start here in Texas and typically will get up to about 130. I thought about replacing it but these numbers seemed pretty spot on

 

They read OK

Maybe your fuel pressure is dropping under load, PITA to rig up a pressure gauge you can see when driving
But can change the fuel pump on speculation

Last stop will be Computer issue

 

RonD, thank you for all your help


I wouldn’t be surprised at this point if it’s the computer, there was one time at a light when I was starting to slip the clutch in first and you completely had no power I could barely get it to turn into a parking lot out of the way and it was shaking the whole truck super bad. Turning it off and on basically fixed it and hasn’t happened since

today watching the fuel Trims it seems to be about 14-20 at idle and then while cruising they are about 10 and under throttle 20+


how much do junkyard computers run?

Do you know which will match mine ?

2005 3.0 2wd manual trans

 

Unfortunately in 2004 and up you can't just swap computers
Rangers got a Digital Dash, HEC Cluster, that is "married" to its engine computer, so people could no longer swap in lower mile odometers

Forscan software may be able to "re-marry" a new computer, check at Forscan forum
Your 2005 doesn't have PATS so that part is not an issue for this one year