I have a 1998

!998 Ranger Stepside Xlt 104k miles 4.0 five speed
I have this great Ranger low miles 104k, in fact it sits garaged and we pull it behind motorhome when we travel........ It runs and drives great! But I been getting P0171-P0174 If it didnt throw the codes I would never know something was wrong.
I have replaced everything that could cause this code. MAF, O2 sensors, did smoke test no leaks even pulled the Intake off and replaced lower gaskets fuel gaskets and Upper even new o rings on fuel injectors. Tail pipe is clean no carbon. I run scanner when driving and Shrt term Fuel trims are 22 to 25% but here is the confusing element. I can start truck and sometimes after coming out of Ol to Closed loop the trim levels average 0 to 10 to 15% at idle, with scanner on as I drive It will stay normal then all of a sudden may 5 -10 miles then short term trim levels will go as high at 20% to 30% and stay that way. Then after driving more they will settle back down to normal. Sometimes when I start while scanning live data and driving back up it and goes CL it will just go to 22% plus percent then slowly it may go down to 15%. to 25% At the same time the truck runs fine.

If it didnt throw the P0171 P0174 code I would never know their is a issue. I been dealing with this and it drives me nuts because the truck in mint condition inside and out and I want it to run the way it looks. whats also weird and maybe because I dont understand everything is when it does this and give it gas LT trim levels dip off the 25% they are locked at down to single digits, then take foot off gas then goes back to their high levels 25%. What can cause this intermitant fuel trim swings. Can it be a EGR valve sticking or not closing all the way? I have replaced even the fuel pump, filter pretty much any part that would have a effect on these codes. I didnt mind changing out everything in this process because my thought was its 25 years old and I used all motorcraft parts.

I just want to figure it out any help or more suggestions would be great,

 

1998 was first year Rangers were switched over to Returnless Fuel system, so computer expects 55-65psi fuel pressure at the injectors
(1997 and earlier used 30-40psi)
You didn't mention testing fuel pressure, there is a test port on the fuel rail, looks like a tire's air valve

Computer bases fuel delivered on injector size and fuel pressure, both are pre-programmed parameters, there is no fuel pressure sensor, so if either changes you can get lean or rich codes
Lean(or rich) codes don't mean engine is running lean(or rich) it means computers calculation for injector open time was off, so if fuel pressure was lower than say 40psi less fuel flows out of each injector, and computer registers that as lean and opens injectors longer to compensate, +25% means 25% longer than calculated, computer sets lean codes if fuel trims are consistently over +15%(or -15% under, rich)
Engine runs fine during any of this because computer is adjusting injectors open time from O2 sensor feedback

Out of range Fuel pressure would, of course, cause codes on both banks of a "V" engine

Its possible its just a dirty fuel filter but more likely to be a failing pump or Fuel Pressure Regulator(FPR)
1998 may still have the FPR on the fuel rail, looks like a small can with a vacuum port and the one fuel line connects to it, follow the one fuel line that comes into the engine bay it will connect to the FPR

This was dropped after a few years because of issues and FPR was moved inside the gas tank and 3 port fuel filters were used

 

Yes I am t 60 psi plus I replaced fuel pump and fuel filter thinking maybe original Fuel pump that was causing pressure to drop and then bounce back causing my up and down fuel trims. The fuel regulator is on fuel pump But on fuel rail is a item that looks like a regulator but it is on a vac line is a fuel pressure damper Thanks for responding.

 

So you have a 3 port fuel filter on a 1998 Ranger?
Odd, although FPR could have been put on the output of fuel pump inside the tank so no external return line

Yes, if the single fuel line is not connected to the device on the end of the fuel rail its a Pulse Damper

The lean/rich part of the system is not to complicated but does have several sensor inputs
Everything is based on upstream O2 sensor feedback, and on a "V" engine it makes it less complicated because if BOTH upstream O2s are showing Lean burns then problem has to be something common to BOTH upstream O2s

Computer knows its running a 4 liter engine so knows exactly how much air is coming in at any RPM and throttle position, its just math and that's what computers do well, they compute
MAF weighs the incoming air for elevation and density
Air temp sensor also plays a part, cold air is heavier than warm air but also based on elevation
Computer knows injector size and pressure at each injector, so knows exactly how much fuel will flow out if an injector is open for say 100ms(milliseconds)
Computer does its calculation based on above and that's 0 STFT

Then the upstream O2s come in
0.1 volt is high oxygen, lean
0.9 volt is low oxygen, rich

If 0.2v is seen then computer changes open time to +2 STFT, 2% increase, 102ms open time
If still 0.2v then +4 STFT, then +6 STFT, ect...........................
Until O2 shows 0.4 volt or so
All this takes less than 2 seconds

Vacuum leaks cause lean codes because MAF sensor is reporting thinner air(lighter air weight) based on flow of air into the 4 liter engine which is a known flow rate
A vacuum leak would have to be in upper intake to effect both banks
The air plenum from MAF to upper intake can also be a leak point, so same as a vacuum leak but its not vacuum, ALL air must pass thru MAF sensor or it will be off, light, so lean

Air temp doesn't effect calculations too much since it gets fairly stable after intake warms up, but if sensor is reporting higher temps than accurate then it could cause leaner running

Fuel pressure of course as discussed

Exhaust pressure, there should be none, lol, a partially clogged exhaust will cause back pressure which an O2 can see as lean since oxygen is not being blown out fast enough
But there should be poor running at higher RPMs if exhaust flow is not good

Exhaust manifold leak can cause lean as air is sucked in, but only effect the one bank of the "V" engine

If you have gone thru all these then computer is the issue, its not computing correctly

You can use an OBD2 reader and check that all the sensors are reporting data within correct ranges in Live Data engine running and driving
If sensors are all correct then computer is the issue

A stuck open PCV Valve is an air leak, often overlooked

 

Thanks Ron I will recheck all the levels on my scanner again in real data and see where they are. I have done this in past but recheck again when short term trim levels jump up from normal to 22 plus % All the parts have been replaced you mentioned. including all the non colored vacuum hoses. But I will recheck.

The fuel system is a non return system. 2 ports on fuel filter. with the fuel damper located where fuel line feeds the fuel rail. If all the sensors are reporting the correct levels Ill find a used computer and drop it in,
hell I replaced everything else. Thanks again

 

Has to be from a 1998 4.0l with matching transmission
1997 PCM has wrong fuel pressure
1999 4.0l PCM used 2 wire Cam sensor, no interface to wire around that

If fuel filter only has 2 ports then FPR is in the gas tank on the output of fuel pump, this system did not work well for a stable fuel pressure at the injectors
Ford switched to 3 port filter, 3rd port is return line back to gas tank with FPR in the tank, but on the return line, this gave the engine a more stable pressure

I think in 2000 is when they started to switch over, 2001 and up all used the 3 port filter
Its possible this is the issue, pressure is dropping then coming back up, you would need to install fuel pressure gauge so you could see it while driving

 

You are correct and thats what I got in my 98 and new fuel pump and pressure regulator in tank.

Ford also changed the Cats on the Y pipes mid year on the 98 and the original set up had the cats after the Y pipe.

That said, Ford stop making it and no after market Y pipe is available. I had a few smaller holes in Y pipe that was causing a leak cats came after Y. They tried to get original Y pipe off to get welded (cost really wasnt a issue. I wanted it fixed but we could not break the bolts holding the pipe to maniford. Not matter how much heat we put to them. I was able to apply this product Iwrapped the Y pipe as instructed. I used 2 applications being its around 2 pipes coming together to Y and wrapped it like a cast. As it heats there is a clay in the fiber tape that turns it into a solid rock hard cast. It had great reviews and I have to add this worked great. Plus its not pricey that if it didnt hold in time its a easy application. Its been well over a year and the fix is still rock solid. The rest of the exhaust system is new from cats back no leaks.

 

Only exhaust leaks that can cause False Leans have to be between head and upstream O2 sensor, leaks after that wouldn't cause lean codes

 

Hi what was the final resolution to this? As a novice, am I reading the procedure to diagnose P0171:

clean MAF
Check vacuum leaks
Check fuel pressure (this is where I’m at)
use OBD2 scanner for live data?

This isn’t the easiest code …. Seems like a flowchart is needed.

 

You need to think like the computer...............................and these types of computers don't "think", lol

P0171 System too Lean (Bank 1)
A 4cyl engine just has Bank 1
A V6 or V8 engine also has Bank 2, so if you have a V6 engine then that's a clue that ONLY one side(bank) of engine is effected
Which eliminates any thing they share in common, like MAF, upper intake and fuel pressure

If its a 4cyl engine then those are still on the table

Lean code doesn't mean engine is actually running lean, if an engine is pinging or knocking then its actually running Lean

P0171 means computer's calculated fuel mix is Lean, so its having to add at least 20% more fuel to that bank to get O2 sensor on that bank to read in the 0.3 to 0.4 volt range

O2 sensors are the ONLY sensors that wear out, 100k miles or 12 years, after that they are costing you money and will eventually cause Lean codes to be set, but its probably cost you $500 in extra fuel cost by then, lol, so best not to wait for the code

In any case the O2 sensor is the only feedback the computer has on if its Fuel Trims(calculations) are OK, so if O2 is running out of chemicals then you are getting False Lean and wasting money on fuel or "fixes"

Computer bases Fuel Trims on ALL the air coming into the engine passing thru the MAF sensor, MAF sensor WEIGHS the air passing by it, computer already knows the Volume of the air, i.e. if it programmed for a 3 liter engine then it knows exactly how much air is being pulled in an ANY RPM, its just math\
What it doesn't know is how much that air WEIGHS, which changes quite a bit in cold and warm weather and in low and high elevations(above sea level)
If a MAF gets dirty then it reports "lighter" weight air, so computer adds less fuel and................lean calculation
If there is a vacuum leak, air leak, then MAF is reporting "lighter" air again, so..............lean calculation

Fuel pressure
There is no fuel pressure sensor in Rangers
Computer is programmed for injector size(flow) at XX fuel pressure at each injector
1997 and earlier computer "assume" 35psi fuel pressure
1998 and up computers "assume" 60psi fuel pressure

If pressure is 20% less then calculations are off by 20% and...............Lean code

Except for the O2 sensors, other sensors rarely fail/wear out, but are often replaced, lol
Its a good idea to unplug and plug back in any sensor you come across as that "cleans" its connections, and why a new sensor fixed a problem, you had to unplug it to plug the new one in

My 1994 has all its original sensors except for the O2s

Also when doing any work on the engine unhook one battery cable, yes its for safety but ALSO to reset computer, if battery is unhook for 5 min or so, computers "learned" memory is cleared, when its hooked back up to power the computer will reboot Factory software and "re-learn" sensor and control(IAC Valve, EGR, ect....) settings

 

This was really helpful. Based on my truck history, and a peek at the rusty globs posing as 02 sensors, ill assume they’ve never or less bf ago been replaced - and start searching for tips.

It’s a 4.0L 99 with 180k. Thanks again for the clear explanation.

 

Yes, I would get new O2s, at 22 years old and 180k miles it should be almost time for its 3rd set of O2s in any case, so NOT a waste of money