P0171 And P0174 Lean Codes After FlexFuel Disable

[BamaFootball]

Hey Guys,

I am running out of ideas and would appreciate any insight you may have.

I have a 2011 GMC Sierra 5.3L and it has the occasional Ethanol Fuel % number that goes crazy high (59% this last time) and starts causing cranking issues. I have never used anything but regular 10% Ethanol fuel in it. Other than this my truck runs perfectly. 100% stock with no crazy mods.

I have been researching the problem for quite some time and want to fix it instead of taking it into the shop where they just reset the % back to 0 and it runs fine for a few months for $100. I bought HPTuner and have made the following changes. Disabled AFM, Disabled the Master Switch for Flex Fuel, changed my Stoich tables to 14.7 across all values. Things seem to run fine now but after a few trips my vehicle throws DTC Check Engine codes P0171 and P0174 System Too Lean Bank 1 and 2.

I have checked for vacuum leaks using carburetor cleaner, cleaned my MAF and numbers look fine.

Is there anything else that I need to do in addition to my steps above to successfully disable Flex Fuel?
Are those error codes valid or just a symptom of my changes that now need to be disabled themselves?

Any help would be appreciated as I am stuck.

[5FDP]

Stoich for E10 fuel would be 14.10, I would still correct that.

Those codes can point to another problem and should not be disabled.

You can check fuel pressure and make sure you have no broken exhaust manifold bolts. Broken bolts can let oxygen into the exhaust.

[blindsquirrel]

The 'too lean' condition is what was causing it to think it had high ethanol content all along.

[BamaFootball]

Damn good call 5FDP. I do have a drivers side rear manifold bolt broke.
I would have assumed that this would have only affected that bank and not both sides so I ruled that out.

Is there some parameter that I can check that would identify that side’s pressure loss or something?

[BamaFootball]

I think you are right.
Thanks for your input.

[blindsquirrel]

Log, with all the goodies in it (IPW, MAF data, short/long trims, O2s, EVAP state, etc.), preferably while it's progressively adding more and more fuel via trims until it gives up the codes reset. Now that the uncertainty added by the stupid 'virtual sensor' is gone it can be diagnosed just like any other lean code.

This is a snippet from the Fuel System Description & Operation in the MY2009 manual (the most recent I have, did not change from this through 2011):

After the re-fueling event, the system registers the amount of fuel that was added, relative to the amount that was in the tank. Reading fuel trim and O2 sensor activity, the system determines if the fuel added was either ASTM Gasoline or ASTM E85. Based on that determination, the system adjusts to the expected alcohol mix in the fuel tank, and then the fuel trim and O2 sensor activity fine tunes the adjustments. The system must remain in closed loop in order for this adjustment to occur. Numerous short trips after switching from gasoline to E85, or E85 to gasoline, can result in driveability symptoms due to the inability of the system to adjust for fuel composition by not attaining closed loop operation.

So it thinks it is really really clever but it's just guessing, and it is not smart enough to know if it's being fooled or not.

Your very same truck in the pre-flex-disabled state, with a different driver doing a different type of drive cycle and different refueling frequency, could have had the truck pop the P0171/174 instead of taking the fuel trim data and using it to miscalculate the ethanol content. It was just a fluke or whatever, something about your driving routine, that the lean condition showed up as ethanol content errors instead of lean codes.

[BamaFootball]

Good stuff.

Thanks BlindSquirrel