Hello team. I've got quite the head scratcher and I'd like you to come on the journey with me if possible. I'd like to chronicle whatever the fix is here so folks can learn from my mistakes.

First of all, all of the other P0171 & P0174 threads here have gotten me to this point - but I do have some questions for the pros and I have certainly not fixed the issue yet so there is more to learn and do.

Let me set the stage:

• 2004 Ford Ranger XLT 3.0L 4x2 "U" VIN (Non Flex) OHV Automatic
• P0171 & P0174 System Too Lean in banks 1 & 2

What I am observing:

• Short term fuel trim for Banks 1 & 2 registering between 10-17% at idle in closed loop
• Short term fuel trim RAISES with the RPM - Question #1; does this mean it's a fuel delivery problem?
• Long term fuel trim holding steady at 30.5 (which I know is a symptom of the high STFT)
• With unit of measure on Metric, MAF reads 3.20~3.40 g/s at idle (669' +- of elevation where I live)
• If I unplug the MAF, the fuel trims do drop to negatives at all times accordingly to try to correct LTFT, but sometimes the truck will hesitate to respond to the gas pedal and I'll get harder shifts
• Fuel pressure while running hangs out at approx. 65 psi at the rail
• Vacuum while running hangs out at 18 in Hg.

What I have done so far:

• Checked for flagrant vacuum leaks
  • No hose really stands out as cracked that I have found, I have also ran some brake parts cleaner against many components, joints, etc
  • Unplugging the IAC while the engine is on results in the engine stalling (which is good from what I understand)
  • RPMs hang out around 720~750 while idling
  • I have plugged the manifold port for the EVAP / removing this from the equation with no change
  • I have changed the PCV valve itself on the rear side of the engine
  • The AC/heater/vents do work, so at least on the AC side of the vacuum system, we seem to be in good shape
• Replaced the MAF, twice:
  • Once with the (O'Reilly option) Blue Streak brand, no avail
  • Once with the Motorcraft brand, no change - currently running this sensor.
• Replaced the PCV valve itself
• Replaced the IAC, twice:
  • Once with the (O'Reilly option) Standard Ignition brand, no avail
  • Once with the Motorcraft IAC - currently running this IAC valve
• Replaced the Fuel Pump inside the tank, twice:
  • Once with the (O'Reilly option) Precision Pumps brand, no avail (although this broke my fuel gauge)
  • Once with the Motorcraft pump, no avail (this fixed my fuel gauge!) - currently running this pump
• Replaced the Fuel filter with a brand new Motorcraft filter
• Replaced the air filter with a O'Reilly option paper filter (not that it should matter)
• Replaced the Spark plugs and oil, oil filter
• Replaced both B1S1 and B2S1 O2 sensors
• Running fuel system cleaner through the tank:
  • Currently on half of a tank of Lucas complete engine treatment in the fuel in hopes of cleaning some injectors
  • I have some Valvoline Super-concentrated fuel injection cleaner on deck for the next tank.

So, my questions thus far:

1. Would short term fuel trim increasing with RPMs be indicative of an actual lean scenario / fuel delivery issue? Despite PSI at the rail being within spec?
2. Is 18 in Hg of Vac about right for the 3.0L? Should it be higher?
3. What have I missed, guys? Or not thought of?

Without knowing any better, I'd think it is an actual fuel delivery issue now with the main lead being the fact that the STFT increases with RPM, perhaps due to the injectors? Maybe one or two injectors being dirty on both sides are enough to need to run the injector cycles 10-20% longer causing my issue?

All advice is greatly appreciated, I love this truck and am looking forward to figuring out this head-scratcher.

Have a good day.

RT

 

Made some edits to add a few things that I forgot to add to the "What I have done so far" list.

Best,
RT

 

Looks like you have done very good diagnostics and covered all the bases

These 2 Lean codes mean upstream O2s are reporting higher Oxygen levels in the exhaust manifolds than should be seen based on STFT 0 with LTFT added
STFT 0 is what computer has calculated as the Open Time for each fuel injector with LTFT added
So if LTFT was +30% then STFT 0 would actually be +30%, so STFT +10% would actually be STFT +40%, so computer is opening the injector 40% longer that it calculated to get the correct Oxygen Levels in exhaust manifoild
Point of LTFT is so computer doesn't have to "learn" an older engine systems quirks at every start up, especially on cold start when O2s are off-line

Spark plug tips should be very dark brown or blackish if its a False Lean, because engine is actually running RICH because of "false lean", i.e. bad MAF, Vacuum Leak, low fuel pressure, exhaust manifold air leaks.................

You didn't mention how the engine is running, or did I miss that part?
Misfires don't burn up Oxygen so O2s see that as Lean
Clogged exhaust can cause lean codes, but you would also notice lack of power at higher RPMs

STFT should go high on acceleration, then level off at -5% to +10% when cruising at speed, that is normal

I would also replace Downstream O2s as they will effect LTFT and STFT, but not your issue
Cat converters need unburned fuel to stay hot and working, so if downstream O2s start to show voltages under 0.75v the computer will add more fuel(higher STFT) to try and get the Cats hotter
Downstream O2s should be fairly steady(unlike upstream O2s) and show 0.75v to 0.85v if Cats are working like they should
O2s see Oxygen only
0.1v is high oxygen
0.9v is low oxygen
Cats burn up most of the remaining oxygen to burn up pollutants, so low oxygen levels after the Cats, so 0.75v-0.85v

 

Thanks RonD, admittedly much of the bases covered come from your interactions with other members on this forum over time!

To answer some of your questions, starting with how the truck runs and drives: It runs just fine! There is a slight "warble" on startup then things tend to smooth out and after that it's perfectly driveable.

Let me pull a spark plug here and see what the story is - I too wonder if it is a legitimate lean.

Regarding clogged exhaust - it is possible. In terms of lack of power at higher RPMS - I do feel like it could potentially have more power in high RPMs, but I also know the Ranger is no race car. :) I have only had the truck for about two weeks now, so this current state of it is all I have experienced.

Another claim to back up the theory of exhaust related shenanigans, is that when inspecting the O2's - one of my downstream sensors are actually WELDED on to the bung. Beats me as to why someone would do this instead of just welding a new bung, and doing it correctly!

I can't imagine that sensor is doing it's job - could that be the culprit? One of the downstream sensors being dead? I did try to unplug that sensor for a day after I noticed it to see if I would gain any new CEL's based on it being unplugged, but it did not make a difference in how the truck performed nor cause any CELs.

With my current view of the diagnostics, using an Autel AL319, I see the following values for downstream sensors:

• O2B1S2
  • approx 0.135v
  • "SHRTFTB1S2" N/A
• O2B2S2
  • approx 0.230v
  • "SHRTFTB2S2" 99.2

So I did google that 99.2 before and someone mentioned it was a-ok, but odd that my O2B1S2 shows N/A (it's possible that this is the one that was welded onto the bung).

Now - this might be the culprit, maybe clogged cats and/or the cats not working? However I took those readings when the truck was cold. Perhaps I need to grab O2B1/2S2's when the truck is warm and the Cats should be warm and operating to look for those 0.75~0.85v readings.

If there are any testing methods here (such as, unplugging both downstream O2's to see if the ECU would fallback on to default values and throw me a code, but thereby correcting my STFT momentarily) I'd love to try and isolate the issue.

Thinking about this out loud - if it were clogged cat / exhausts, my high RPM power would be low (this does feel to be the case, although I don't know how it SHOULD feel admittedly), AND my spark plug tips should be black as the night because the fuel delivery is not the problem, correct?

I'll post this while I go get my spark plug removal sockets... and after checking that I'll let the truck warm up and get downstream sensor voltage.

It's worth mentioning that also I put my hand over the exhaust pipe just now and do feel like there could be more flow/volume coming out of there during idle. Also before I took possession of the truck it was sitting in a field for about a year and a half or so, so it's possible that some little critter found the exhaust pipe and made a home/tomb. Or simply that the cat(s) are clogged for natural reasons...

 

Something else worth knowing based on the information that I failed to provide, is that the truck has just a hair over 101k miles right now (specifically thinking in terms of catalytic converter lifespan issues).

Be back shortly with more info!

Thanks,
RT

 

Here is one of the plugs, not too bad it seems (could be worse). These plugs are about 2 weeks old as well.

 

O2s need to be heated above 600degF to start working, so those readings won't be helpful

If an O2 sensor is unplugged you should instantly get an O2 sensor Heater code, engine warm or cold doesn't matter
Each O2 has an internal heater that gets 12v with key on, the computer has the ground wire for each O2s heater, so computer "looks" for that 12volts passing thru the heater wire with Key on, if its "sees" the 12volts then it Grounds that wire to turn on the heater, if it does not "see" the 12volts then it sets the O2 heater code for that one O2 sensor
i.e. P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

If the O2 was IN the Cat converter, instead of after the Cat converter on an exhaust pipe then it may be part of the Cat so not removable/changeable without changing the Cat converter, which is why it was welded in place

I would expect Cats to last 200k at least, "they" say 10 years, which would be 120k miles, but expected, in my opinion, would be 200+k miles

Yes, spark plug tips tell the tale of actual lean/rich running engines

Also MPG, but with only 2 weeks to go by that wouldn't be possible yet


3.0l Vulcan engine is a HIGH RPM engine by design
Most engines make best power/torque at 2,500-2,900rpm
3.0l Vulcan makes best power/torque at 3,500-3,900rpm
This is why it was nick named 3.slow
If you drive like other engines it will seem gutless at 2,500-2,900rpms, it needs higher RPMs to get into the powerband

 

Alright, so I posted a pic just above your last post of one of the sparkplugs in bank 2 - the foremost plug.

It doesn't seem too blackened by fuel at this point, just a bit browned. Thoughts on this?

Also afterward I replaced the plug and ran the truck for about 20m at idle, then observed the following reading from the downstream O2 sensors:

• O2B1S2
  • Bouncing between 0.315 - 0.5XX voltage
• O2B2S2
  • Remaining at approx 0.165ish / below 0.2 volts regardless

This is with 20m of idle time, and the engine at operating temp (195 or so), am not sure if that was enough time/activity for the cats to get to 600F.

Also it's worth noting that with this P0171 & P0174 - if I clear the code, it will come back on nearly right away. So I think this condition happens prior to the engine reaching operating temp (if that is helpful to know).

Let me know your thoughts on the spark plug image, if that's a true lean or that's enough brown to call it rich / false lean. Also the thoughts on the "warmed up" O2 sensors and if that data is usable.

Later today I am expecting a package delivered that will have a better OBD2 scanner so I should be able to get time series data instead of sample frame readouts.

Appreciate all the help Ron, by the way!!

Thanks,
RT

 

Spark plug looks good, prefect in fact, light brown means good air/fuel mix and burns

B1S2 is usually the only downstream O2 sensor on single exhaust systems
Its voltage means either the O2 is too old or the Cat(s) are
I would change the O2 first as its cheaper, lol

See if a new one runs between 0.75v and 0.85v that means Cat(s) are OK
O2s use a chemical reaction to generate their own voltage, after 12 years they start to run out of the chemicals so voltage slowly goes down, takes a few years, to get low engine to set codes
But starting to suspect computer if its not throw a code for low voltage on B1S2

Computer probably doesn't have a B2S2 hookup, OBD2 readers do not know what sensors are on any engine, they rely on the user to know that
I only show 3 O2 sensors on my 2004 3.0l wiring diagram
B1S1 upstream right
B2S1 upstream left
B1S2 downstream right
No B2S2

 

Good to know! That's gotta be worth something. At least I know the ECU is correcting for the condition and it seems ok to drive for the time being while figuring it out.

I will put a new O2 sensor in downstream tonight. If I get the same reading with a new sensor on a warm engine (less than 0.75 ~ 0.85) then we can assume the cats should be replaced, correct?

So let me go through my understanding... we are burning GOOD per the spark plug, but fuel is having to be ADDED because the cats may be blocked and are unable to properly heat up and do their job?

For us to have a sane burn with all that extra fuel trim, we must assume the cats are blocked right? And therefore we are getting incomplete exhaust flow per combustion cycle leaving air in the cylinder ?

And this also explains some "straining" / loss of power in high RPMS that I may be feeling?

Let me know if I'm mentally on track here and I'll update tonight after a trip to NAPA for the o2 sensor :)

Thanks,
RT

 

Yes, if the new downstream O2 is still under 0.75v then quite possible Cat is at end of life, and if RPMs are limited then it also could be collapsed inside

The spark plug tip looks like good burn
But the STFT is going pretty high to GET the good burns, should be -5% to +5% when cruising a long
So Computer "thinks" it needs to add more fuel based on upstream O2 sensor feedback, but spark plugs don't show "False Lean", i.e. Richer Mix, which is puzzling

At idle +10% is not unusual computer has to run engine richer at idle or it will over heat
Accelerating, +15% or higher is OK, depends on how fast you are accelerating

But 2 weeks is not a long time on the spark plugs
Do you have the old ones?
Or do you remember what they looked like?

 

Agreed! This is quite the puzzle. Ok, to answer your question regarding the plugs - unfortunately the guy I bought the truck from changed them so I was never able to see how they looked prior.

The good news is I got my new OBD2 scanner today which allows me to see time series data, so I will post that.

Beforehand, I wanted to share some pictures of the truck and sensors. This 2004 3.0L does indeed have 4 sensors as opposed to the three found in the diagram.

I have annotated each picture to show bank and sensor position:



Bank 1 Sensor 1

Bank 1 Sensor 2

Bank 2 Sensor 1

Bank 2 Sensor 2


So of the four sensors, I replaced Bank 1 and 2’s S1 sensors and also Bank 1 Sensor 2 as seen in the photo.

I was unable to replace the welded Bank 2 Sensor 2 due to the weld, but at least now you can see it in the picture.

The P0171 & P0174 still exist for the time being, and now I can show you time series for all of the data with my new OBD2 scanner… it does tell a tale:


Fuel Trims Bank 1 & 2, Short and Long

O2 sensor time series position 1

O2 sensor time series position 2


So according to this thing, 3/4 O2 sensors are hitting .7 volts (and see-sawing back and forth). I have referred to “Bank 1” as driver side, is that true with the Vulcan? If so - it looks like either the NGK sensor I put in B1S2 is faulty, the pre-cat for B1 is blocked, or I am mistaken and the passenger side is Bank 1 on the Vulcan and the bad sensor is the welded one.

I can try to unplug a sensor tomorrow to confirm bank position… I’ll leave this information here for the night and catch some rest.

Thanks for looking at this with me, Ron!!

Cheers,
RT

 

One more piece of information that you may know about this truck but maybe not, as I too just had to look and think about it is the cat structure.

There are 3 catalytic converters: the two pre-cats (one on each bank in between sensors, and a third cat that both pre-cats join into.

The fun fact is after the 3rd cat, there is no sensor.

So one theory is that maybe the 3rd cat is blocked/collapsed since I’m reading both banks lean and that is impacting both pre-cats?

I would think that if one pre-cat was toast, I would only have one bank reading lean… but who knows!

I might drop the third cat tomorrow or Friday night and see if the truck runs much better without it for fun… that would allow me to inspect that one, test the engine in high RPM, see what happens with the fuel trims… and be interesting to hear

Of course this wouldn’t account for B1S2’s 0.4 voltage reading, however… but hey it’s an experiment nonetheless.


Have a good evening Ron!

Thanks,
RT

 

Bank 1 is passenger side, where spark plug/cylinder #1 is located
Bank 2 is drivers side
On all Ford V6 or V8 engines

On transverse mounted V6 engines Bank 1 is the back side, Bank 2 the front side, just FYI

Tap on Cats, or mufflers, with a wooden stick and listen for internal rattles, if heard then parts inside have separated

 

Sounds good, I’ll do so tonight and see if I can. Also I ordered a back pressure tester gauge to see if I need to replace both the pre-cats. The third big cat behind the second position sensor looks like it would come out real easy to test / inspect.

I mention needing to test / inspect tonight if I can because some rain just rolled through and we’ll take it. I don’t have a garage at the moment, just working in the back porch and yard.

I also got another o2 sensor for downstream testing because of the one above giving the strange results with low voltage.

Admittedly I need to learn about downstream voltage and what to expect, but also I drove the truck around 80 miles today and do think the top end acceleration could be much better than it is, so I expect that we will find a clogged cat for sure.

Will keep you updated on the cat discoveries!

Thanks,
RT

 

Regular vacuum gauge can test for clogged exhaust
Hook up gauge to intake
Raise and hold RPMs at 2,500-3,000, steady
Watch the gauge, if its slowly dropping you have exhaust blockage
If its slowly going up your air intake is clogged

 

Well well well - I can test now then

I got a manometer, will hook it up and give it shot.

That’s good info, and I’ll report back soon!

Thanks,
RT

 

OK Ron,

At idle on startup I have around 19in Hg of Vac today.

When I rev to 2500 in park, it shoots up (or down, depending on how you look at it) to 25in Hg of vac.

I wouldn’t call it slow though, it pretty much follows the RPMS there.

I did hold RPMs at 2500 for a few seconds and the gauge pretty much stayed consistent at 24 or 25 in Hg of vac during those times.

The weird thing is after this test, my STFT temporarily have gone to… the right values? Right this second they are showing around zero and slightly negative fuel trim. See below





That little exercise seemed to beef up the vacuum pressure and maybe that temporarily solved it?

I do believe if I turn the truck off and back on its likely to go back to its original behavior…. Is there a logical explanation here? It’s taken me about 5 mins to type this up and the fuel trims are still holding around the 0 mark.

So I ran a small test and hit the throttle again and it went from behaving at 0.0 STFT back to trimming upward, see below:




Will post this progress right now to see what you make of this madness!

Thanks,
RT

 

This made me think about getting my fuel pressure gauge and doing a similar test…. Because one thing the truck has always done is the STFT follow the RPMs upward.

So here we go, same test but with Fuel pressure at the rail.

From engine and ignition in the off position, to Key on Engine off - the fuel pump builds from 0-55 psi at the rail on one key rotation.

Key on Engine Running - seeing the fuel pressure at the rail 65psi.

If I blip the throttle there’s a momentary drop to 60psi and a rapid catch-up to 65psi at the rail. If I hold RPM at 2500 the fuel at the rail will stick solid at 65psi.


So at least to the rail, the fuel system seems to be performant with the new Motorcraft fuel pump and fuel filter…. I’m still racking my head to understand how both banks are being affected, but unless there is like 1-2 dirty injectors on each side, I think this test speaks toward the fuel systems health.

Also I filled up today and put some Valvoline Super Concentrated injector cleaner in this tank. Next tank will use Techron.

When the ground dries, I would still like to remove the rearmost Cat and inspect it / run the engine without it but do let me know what you think about the vacuum gauge test.

Cheers!
RT

 

Can you reset LTFT?

Or does it still show 30% at key on?

 

On the KAM reset - I will look at doing that tonight, the good news is that I did drop the rearmost cat and can definitely see that they have fallen apart on the inside.

I can see Bank 1 pre-cat has degenerated / broken apart as well, Bank 2 makes a 90deg turn so that one is most likely broken too since it’s done all the things that bank 1 has done.

That said, I have ordered the bank 1/2 cat assembly from magnaflow and should have it in house tomorrow.

This really does explain how the power is gone at the top end, I am really looking forward to fixing this :)

Now let’s hope it resolves the fuel trims as well!

Also I will post some pictures when everything is off.

Cheers,
RT

 

Alright I did just clear the KAM and LTFT does start to build up as she wants to trim up 50ish percent.




The LTFT is learning here so it’s shifting up and down on its way back to 30.5 I suspect!

Either way I am looking forward to getting this engine to breathe better and really feel the power increase.

More on that soon!

Thanks,
RT

 

There is no feedback with no O2 sensors
STFT and LTFT are calculated based on O2 sensor feedback only, there is no other way
So those numbers might as well say "taco" or "green", they are not real "numbers"

 

That is true, I didn’t try to drop the cat and take any fuel trim readings. That capture from above is with the current cats installed and just a KAM reset driving on the highway to work.

I am going to pull the cats again tonight and post pictures, tomorrow I should have the magnaflow bank 1&2 cat setup… and I am quite excited about it.

You don’t often get a chance to change like 4-6 bolts and make 20-30 more horsepower, but I really think that’s what’s going to happen with the cat replacement!

Thanks,
RT

 

Okay well hard to see what’s going on here in the pics, but it’s Cat 3 the downstream cat, and it jingles like jingle bells when you shake it.






I was planning on just clearing out the insides of this thing and reinstalling, but ended up snapping that bolt there in the middle so went ahead and ordered the new magnaflow version of itself as well.

At least it will be done right!

Cheers,
RT