P0171 Code Help

[RedRangerSTX]

I have a '02 Ranger FX4, 4.0L, 5spd with about 160k miles on it, and have recently been getting a CEL for P0171 (Bank 1 Lean). The strange thing is that it is NOT throwing a code for Bank 2 being lean. So far I have replaced:

• MAF Sensor
• Fuel Filter
• Upstream 02 sensor on Bank 1 (Bosch)

I also sprayed carb cleaner around the vacuum lines and around the intake manifold gaskets to listen for changes in idle speed, but heard nothing. I would think if either the MAF were bad or the fuel filter were plugged that I would get the P0174 code along with the P0171 code, but the filter was due anyway and I got a good deal on the OEM MAF (not a cheap chinese ebay one). I would also think that a vacuum leak would cause both banks to be lean rather than just one.

The truck seems to idle smooth and go down the road fine. Once in a very great while I will get somewhat of a buck/skip at partial throttle going down the highway. It will do it once, then not again for a long time.

I know these motors are notorious for intake manifold gaskets, but I hate to tear into that job and not have them be the issue. Seems like a bit of a PITA to do...

I do have a buddy with the Auto Enginuity scan tool and software. We are both somewhat new to the software, and aren't really sure what to look for in all the systems that you can monitor. We did notice that while the Input Voltage to the Bank 2 02 sensor was steady, the Input Voltage to the Bank 1 O2 sensor was all over the place.

Any thoughts? Any insight on things I could monitor in the AE software to pinpoint this issue?

 

[cvar]

Since it's only 171 and not 174, I'd guess a gasket leak on the passenger side. Could be intake (upper or lower) gasket, or fuel rail gasket, or injector O-rings.

 

[RonD]

STFT= short term fuel trim
LTFT = long term fuel trim

STFT should go up and down but be between .2 and .8v
.5 is the sweet spot the computer adjusts the injector dwell to get.
Fast changing numbers means O2 sensor is good.

LTFT is what will set the CEL for lean if it is indeed lean.

Yes, if MAF was acting up or fuel pressure/flow was below "normal" then both banks should show lean, BUT...........always a but, lol, both might be lean but only 1 is lean enough to set a code, check LTFT

Leak in exhaust manifold can suck in air, leaning out the exhaust, so lean code is set but not really a lean mix, just a lean exhaust(lean = more air than there should be).
Vacuum leak in intake manifold/head can lean only 1 bank

Dirty injectors can lean only 1 bank
Can of seafoam in the gas tank can fix that


Google: reading LTFT

Read some of the articles, that will give you the general idea of what to look for, and that will make finding the problem easier.

Do NOT swap out sensors, they rarely fail.

 

[RedRangerSTX]

Thanks for the suggestions!

I feel there is a strong chance that this PCV elbow may be causing my CEL...

Going to pick a new one up tomorrow and hopefully that solves the issue.

 

[RonD]

That would be the problem.

I seem to read about that PCV elbow leaking on the 4.0l SOHC alot

 

[adsm08]

That would be the problem.

Well, it's certainly A problem.

 

[DoctorRoboto]

Hey where did you find that elbow? I've got a new '98 4.0 throwing P0171 and P0174 and I'm trying to see if that's the culprit.

 

[TxGasRacer]

on top of Drivers side valve cover near the rear of the engine is where you will find the PCV valve and hose.

 

[RonD]

Hey where did you find that elbow? I've got a new '98 4.0 throwing P0171 and P0174 and I'm trying to see if that's the culprit.

1998 is a 4.0l OHV engine, so not the same setup as 4.0l SOHC engine in this thread, so you won't find that elbow on your engine