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I own a 2013 Suburban 2WD, 5.3, Automatic, LT, 285,xxx miles.
I found I had a problem by accident last year. I run E85 all the time $1.59/gal (Unl $2.39). I stopped at a local gas station and put a couple of gallons of unleaded (87) to get me to a station with E85. After about a mile the engine lost power, started backfiring thru the intake. The engine would not idle. Engine rpms would not go over 1,000. Engine light comes on. Engine has no power to move the truck. I put in 10 gals of E85 and after a few mins it started to run fine. I thought it might have been a fluke. After a few tanks of E85 I let the low fuel light come on and put in 5 gallons of UNL at a station that also had E85. The same thing happened. The code from Onstar are emission codes. P0171, P 0174, P015B.

Over the past year I have replaced the following:
(2) AC Delco 213-4229 GM OEM Heated Oxygen Sensor,
(2) AC Delco 213-3866 GM OEM Heated Oxygen Sensor.
(1) MAP Sensor
(8) Spark Plugs and wires
ACDelco 22963841 GM OEM Vapor Canister
ACDelco 19257603 GM OEM Evaporative Emissions Canister Vent Valve Solenoid Jumper Wiring Harness
ACDelco 23481275 GM OEM Vapor Canister Vent Valve
After all this, I can still replicate the problem. According to the local dealer, parts store and internet there is no E85 (Alcohol sensor). Now the dealer wants to reflash the ECM.

I will add a few weeks I tried this. I only put in 5 gals of E85 in the tank instead of 10 gals. The engine started to run better but not normally. After about 10 miles the engine lost power, started backfiring thru the intake. would not idle. The engine rpms would not go over 1,000. Engine light comes on. Engine has no power to move the truck. I put in the other 5 gals of E85 and it started to run correctly. After half of a tank the engine light goes off. The codes from Onstar are emission codes. P0171, P 0174, P015B.

Today I plugged in a FIXD OBD2 Code Reader (live gauges mode) and noticed the following with a ½ tank of fuel: Ethanol Fuel 69.0%, Fuel Tank Pressure Sensor 16,xxx Pa, Evap Purge (Command) 100%.

While driving the Evap Purge (Command) cycles between 0 – 100%. The Fuel Tank Pressure Sensor reads around 14,xxx - 16,xxx Pa. With the engine running I took off the gas cap and the reading never dropped below 14,000 Pa. Should it have dropped to 0 PA?

I did notice the Fuel Tank Pressure Sensor reading 4-10 a few times immediately after a fill-up. Possibility fuel hitting the sensor.
Should I replace the ACDelco 13502903 GM OEM Fuel Tank Pressure Sensor and ACDelco 20893611 GM OEM Fuel Tank Pressure Sensor Cover?

Any ideas?
 

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Your maf is reading 6gs at idle? What's your rpm? I'm used to hg... And non eth So not much help other than try unplugging the maf, see how it does then after a bit of a (learning) trip also, if the tank pressure did not change noticably whilst removing the cap, that could indeed indicate such an issue (did it hiss or push against you?). Might want to try it with the purge blocked at the intake(to eliminate possibility that purge is constantly pulling vacuum, thereby never building pressure) also whilst watching scanner unplug pressure sensor, note changes. (You can just reach it)
 
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