"Alternating" P0174 lean codes, Bank 2 only, then 1 & 2...??

[fixizin]

Before posting, I studied many of the P0174 threads, and understand both the leaking hose angle, and the ageing O2 sensor scenario... but I've got a "pattern" that might make the proverbial light bulb come on for the RKIs 'round here:

First couple times CEL came on, code reader said "Lean, Bank 2", but NOW, it alternates perfectly between P0174 on Bank 2 only, Clear Codes, drive 5 miles, P0174 on Banks 1 & 2, without fail, it's always Bank 2 only, then 1 & 2, clear, drive, REPEAT... Any insights?

Also worried about that sneaky hose UNDERneath the intake manifold--can't get to it to test or replace.

 

[RonD]

A little confused

P0174 System too Lean (Bank 2)
This is just Bank 2 drivers side of "V" engine and upstream O2

There are other codes for bank 1 and other codes for Downstream O2s

i.e.
P0171 System too Lean (Bank 1)
P0157 02 Sensor Circuit Low Voltage (Bank 2 Sensor 2)

What are the other codes?

 

[fixizin]

(*Foghorn Leghorn ON*) "Ah say, ah say, you ah cah-rect suh!"... I blame mah ageing eyes... plus it's an older reader with a lo-res dot-matrix display... and don't forget that So-Fla GLARE!...

But yes, P0174 alone, followed by P0174 and P0171, back to P0174 alone, etc... like a... psychic sine wave, lol...

PS: Engine seems to be running and idling fine, MIGHT be a bit laggy on acceleration... will road-test if further characterization req'd...

 

[RonD]

If you don't think there is a vacuum leak in upper intake and MPG is not down(false leans) then I would test fuel pressure next
60psi is expected
Assume fuel filter has been changed

 

[fixizin]

Fuel PRESSURE: will borrow gauge this weekend...

Fuel FILTER: been awhile... brands to seek out? brands to AVOID??

PS: ever seen that where it oscillates between one particular bank, then BOTH banks?... cue Twilight Zone theme music...

 

[RonD]

Alternating Lean on a V engine wouldn't be that uncommon , not a symptom of anything specific
Lean code comes when Short term fuel trims(STFT) reach +15 to +18 for longer than a few seconds and then repeat that a few times
On some computers its +20 STFT

So an air leak, blocked exhaust, marginal fuel pressure could all cause that
Both banks could be running +14 STFT consistently, which means there is a problem, but only 1 bank may go above +15 now and then, triggering Lean code

If you can see Live Data then look at STFT on both banks, you will probably find both banks are in the High "+" STFT(above +10) and close to the Lean code line for computer to report the issue to driver, i.e. lean code

 

[fixizin]

I've got Live Data on my reader... what's the normal range for STFTs?... at idle?... at 2,000RPM?

 

[Dirtman]

I've got Live Data on my reader... what's the normal range for STFTs?... at idle?... at 2,000RPM?

0%

Slight switching up and down maybe +2% to -2% if you watch it over a graph is normal but balance at 0%

STFT is instant and should maintain 0% unless the long term fuel trim adjustment has been maxed out either high or low. Any major deviation in STFT means LTFT is no longer capable of correcting because a major deviation in short term becomes long term up to a certain percentage.

You calculate your total fuel trim by adding what the long term number is with the short term. Long term at +20% maxed out and a short term at +15% maxed out is a 35% positive fuel trim (check engine light is furious). But long term can be +15% or so and as long as your short term is 0% you're fine.

Long term is learned data depending on where you live, altitude, engine condition etc so the truck runs it's fuel maps based on that learned data. Short term is only for minor instant correction.

 

[RonD]

And just to add

Also look at O2 reading for S1S2, that's the Downstream O2 that checks if Cat Converters are working

It's voltage should be steady, not jumping around like upstream O2s
Should be running a 0.7 to 0.8 volt, so "rich" as O2 sensor voltages go, lol
It will jump now and then, but stay steady
This rear O2 sensor can effect Long Term fuel trims

 

[fixizin]

{PRE-amble: this is when running on E85... }

OK, streamed some beaucoup data... needz me some guidance distinguishing between the various STFT readouts:

SOOOooo... STFT B1 S2 seems to be way out there at 99.2%...??

LET me get some better snaps...

Mega Postscript: switching from E85 to UNL 88 Octane seems to have "cured" issue (well, CEL is off, but haven't checked run-time fuel-trims since then... will soon.)

 

[Orca]

STFT B1 S2 seems to be way out there at 99.2%...??

Values of 99.2% for STFT can always safely be ignored. Per SAE J1979, "99.22%" is exactly what a PID is supposed to report "if this sensor is not used in the calculation". Since this is a downstream sensor ("S2"), it has no effect on the fuel calculations, it's just used to monitor the catalytic converter.

 

[fixizin]

Mr. Killer Whale, WHERE did you find that obscure bit of info??

Thanks in advance.

 

[RonD]

B1 = Bank 1, passenger side of a "V" engine or only Bank on a 4cyl
B2 = Bank 2, drivers side
S1 = Upstream O2 sensor, sensor closest to engine
S2 = Downstream O2 sensor, sensor after Cat converter

Full Dual exhaust "V" engines will have TWO S2 sensors, B1S2 and B2S2
Normal single exhaust "V" engines or 4cyl, with just have one, B1S2

But your voltage on B1S2 is low, 0.30, S2 sensors should be at 0.70 to 0.80 VDC, time to change that sensor or get New Cats

 

[fixizin]

Thanks for the ECM "lay of the land"... helps me convert cryptic acronyms to useful schematics in "mah hay-ed"...

New Cats?... but, but... they're "only" 22 years old... only 80k miles... is E85 bad for Cats?... hmmm... (of its 80,000 mile life, less than 6,000 of that was on E85... just not economical...)

OTOH, I'll bet that O2 sensor is rusted in there TIGHTLY.

 

[Orca]

Mr. Killer Whale, WHERE did you find that obscure bit of info?

As I said, SAE J1979. That's one of several SAE documents covering OBD2 "stuff". Most of it is not what you'd call "light reading", so I serve as an interpreter sometimes in cases like this. "+99.2%" is the maximum value that can be represented for STFT. It's sort of a "flag" to say "ignore me".

As for the O2 sensor voltages, frankly, looking at a single value can be misleading, especially for "upstream" sensors. You really want to see a visual timeline of data (e.g. a graph). And you should also be cognizant of things like engine run time and open-/closed-loop status. But even for "downstream" O2 sensors ("S2" in this case), I've seen them report unexpected values under various conditions. Seeing a 30- or 60-second graph is typically far more useful.