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In this video I have a look at a GMC 1500 4.8 with the Flex Fuel engine and according to scan data it is running rich... REALLY Rich! Pretty common issue with these vehicles and it is a really simple fix. So if you have a P0172 and P0175 be sure to check the alcohol composition first.
Important: The system is initialized by baselining itself on ASTM spec Gasoline (alcohol 0–10%). When an E85 compatible vehicle is built, an ECM or PCM replaced, or if the learned alcohol content has been reset with a scan tool, the fuel system will need to contain ASTM spec gasoline with 10% or less ethanol content. If the fuel in the fuel system needs to be drained and replaced with ASTM spec gasoline, the vehicle will need to run for approximately 11 km (7 mi) at full operating temperature before the system will be fully adjusted to the correct alcohol content. Once system initialization has been completed and the calibration verified to be the below listed calibration part number or later, gasoline with any ethanol content between 0 and 85% may be added, provided it is added in a quantity of 11 Liters (3 gallons) or more, and the vehicle is run long enough to learn the new fuel
After the re-fueling event, the system registers the amount of fuel that was taken on-board, relative to the amount that was in the tank. Reading fuel trim and O2 sensor activity, the system determines the ethanol content of the fuel that was added. Based on that determination, it adjusts itself to the expected alcohol mix in the fuel tank and lets the fuel trim and O2 sensor activity fine tune 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 vice versa, can result in driveability symptoms due to the inability of the system to adjust for fuel composition (by not attaining the parameters necessary to reach closed loop operation)
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49 thoughts on “Chevy/gmc flex fuel: running rich”
  1. Avataaar/Circle Created with python_avatars ken ken says:

    I had a throttle body relearn done to fix the constant problem after cleaning it. I think I may have made a mistake I did a fuel composition reset using X431 on my 2011 5.3 flex fuel suburban , currently have e85 in my tank. Now I have 2 code errors p0171-00 and p0174-00 fuel trim lean system lean bank 1 and 2. Did I do something wrong? I would appreciate any help what to do and proceed. Thank you

  2. Avataaar/Circle Created with python_avatars Marco Garcia says:

    Appreciate that!!! Learned me something new

  3. Avataaar/Circle Created with python_avatars kenneth waara says:

    Hey you had mentioned most cheaper code readers can do this function. Is there any you can recommend. I just purchased one from Amazon and I made the assumption it could do this reset due to price. But it is not able to reset content. Mines at 62% and both long terms are pegged at -30

  4. Avataaar/Circle Created with python_avatars twwtb says:

    Alcohol is not your friend. My 2013 fifteen hunnert has the 4.8 and the same flex fuel sticker as your subject vehicle. Mine is also fahr ingin red but doesn't have a siren. More good info here stored in my memory banks should such a situation arise. I don't see any E gas available around where I live, but it was omnipresent during a recent trip into the heartland.

  5. Avataaar/Circle Created with python_avatars Michael Morrone says:

    What happened the next time that customer filled up? I'm afraid that the next fill up it shot back up to 60-80% again.

  6. Avataaar/Circle Created with python_avatars KimberDad says:

    2007 suburban.. it creeps up to 80% about every 4 months or so… then I've reset it… was up ? Bad o2 maybe?

  7. Avataaar/Circle Created with python_avatars Branden Doyle says:

    My ltft is at 30 after I rest my achole

  8. Avataaar/Circle Created with python_avatars Warner Concepcion says:

    Eric: "That's why alcohol is dangerous"
    Me on my 4th rum and coke while watching SMA: "Wooooooooow…. I feel attacked right now."

  9. Avataaar/Circle Created with python_avatars Asd Asd says:

    REV up the part canon

  10. Avataaar/Circle Created with python_avatars James Washer says:

    Hey Eric, ONCE, I thought I was wrong, but, I was mistaken

  11. Avataaar/Circle Created with python_avatars Matthew Coibion says:

    What is a good scan tool that I can use to do this without breaking the bank?

  12. Avataaar/Circle Created with python_avatars Brandon Kahuna says:

    ethanol needs to be removed from all gas.

  13. Avataaar/Circle Created with python_avatars jgbooms says:

    What type of scanner can I buy to do this reset myself and check my fuel alcohol levels

  14. Avataaar/Circle Created with python_avatars Jerome Burrasca says:

    Great videos! What's your take on Catch Cans, do you like them for Chevys?

  15. Avataaar/Circle Created with python_avatars twistedsledder says:

    ERIC PO451 tank pressure sensor after market fuel pump GMC Sierra 1500 flex fuel that code keeps coming up after install 29 percent alcohol level have seen the evap solenoid code also PO446 which is not coming up now just the PO451 is this a bad tank pressure sensor on a crappy aftermarket fuel pump this is the second one installed under warranty if 5 volt reference is there what could it be causing this code other than a tank pressure sensor that is not working well on this fine GM product 🙄

  16. Avataaar/Circle Created with python_avatars Mohsin Rehman says:

    My LTFT STFT are – 17… MAF 2.6 on idle and with ac on idle 3.5.. egr error 99.2 sometimes egr fluctuating even negative error… O2 sensor 1 no reading sensor 2 reading is .1 to .9… air fuel ratio commanded and measured fluctuating…

  17. Avataaar/Circle Created with python_avatars U1rich says:

    Didn't know they made FlexFuel 4.8s.

  18. Avataaar/Circle Created with python_avatars James Holbrook says:

    Cant scan tools read the vin from the obd2 port and cant it tell what the vehicle is from the vin so why do you need to tell it what the vehicle is

  19. Avataaar/Circle Created with python_avatars FrugalPrepper's Garage & Garden says:

    This is good. I’m my case this makes sense why the problem got better when they switched to E85 as well!

  20. Avataaar/Circle Created with python_avatars Jack Jacke says:

    80% Alcohol must mean its been drinkin! And your bothering it when its on its hangover day! Whats wrong with you Eric? lol

  21. Avataaar/Circle Created with python_avatars Antonio Claudio Michael says:

    Great video Eric O

  22. Avataaar/Circle Created with python_avatars u tuber says:

    the only reason the government wants alcohol in fuel is so you have to buy more gasoline per year. my car will get 150 miles more per tank of non ethanol fuel!!!!!

  23. Avataaar/Circle Created with python_avatars Pete Medina says:

    Well heck that was an easy fix to bad they're all not as easy as this one. Good information about flex fuel GM's

  24. Avataaar/Circle Created with python_avatars Luis Augusto Dutra Benites says:

    Eric, how many scanners do you have in the shop? Wich one has the larger range of makers?

  25. Avataaar/Circle Created with python_avatars Mike Oneill says:

    You the man….

  26. Avataaar/Circle Created with python_avatars Riker 6305 says:

    Pretty country!

  27. Avataaar/Circle Created with python_avatars throttle bottle says:

    i wonder if the MAF or other things they parts cannoned was actually faulty, making it calculate way off, then never drove it long enough at once for it recalculate..

  28. Avataaar/Circle Created with python_avatars l wilton says:

    Clearly the truck was drunk!

  29. Avataaar/Circle Created with python_avatars Chevboy167 x says:

    Had same problem on my 09 Silverado. Alcohol content was in the 30s when I run 91 pure gas since new. Had my mechanic disable comp data sensor. I will never run any ethenol in it. No more problems and get more mpg!

  30. Avataaar/Circle Created with python_avatars Aaron Bisson says:

    I didn't know you could get a tech 2 outside the dealership.

  31. Avataaar/Circle Created with python_avatars Ryan Hull says:

    Love the "Parts-cannon" reference. We need an official SMA Parts Cannon Shirt!

  32. Avataaar/Circle Created with python_avatars Smudgy Content says:

    😅🤣😂🥴🤣😅😅😂😂🤣😂🥴😂😂😂👍👌🤣😁 the cruising around music

  33. Avataaar/Circle Created with python_avatars Grimm's Auto Service says:

    I have shown your old video with the same prob to customers and let you explain what the prob is and not to top off the fuel tank as often as they do. Thanks for all the info and entertainment

  34. Avataaar/Circle Created with python_avatars twistedcells says:

    Got this on 2015 Silverado, cleaned maf and throttle, boom issue gone

  35. Avataaar/Circle Created with python_avatars Auto Manuals says:

    Thanks Eric. Love your videos – subbed and liked.

  36. Avataaar/Circle Created with python_avatars Tom O'Connor says:

    Best move putting the oem maf back and changing the oil. She's on the mend already, probably perked up on hearing the c/w track. Looks a well maintained neighbourhood with trees and shrubs.

  37. Avataaar/Circle Created with python_avatars DurocShark says:

    Tech 2! I'd recognize that ugly beast anywhere!

  38. Avataaar/Circle Created with python_avatars Jason Michaels says:

    Shivvy, keepin the lights on at SMA since 1938

  39. Avataaar/Circle Created with python_avatars Willie Gillie says:

    Curious what makes the truck think it’s getting ethanol when it’s actually getting regular gas?

  40. Avataaar/Circle Created with python_avatars ArcadeAir says:

    I had the 172 and 175 codes intermittently on my 2014 Silverado 5.3 GDI and the first thing I looked at was the fuel composition. It was at 6%. fuel trims were off the charts, both banks ltfts = -35 and stfts = -30. MAF sensor not over reporting either. I isolated the PCV with no fuel trim improvments, then isolated the evap with the same results. The problem disappeared a few weeks later and has not recurred. I am open to suggestions as to what would cause this

  41. Avataaar/Circle Created with python_avatars V. Stag says:

    My muffler bearing is loud I spayed a whole can of wd40 in the tail pipe I didn't help what oil should I use its a ford

  42. Avataaar/Circle Created with python_avatars Terry Stroble says:

    I wonder if some of the Ford flex fuel vehicles can have similar problems? Twice now in the last few years I have had rich codes on my 2004 4.0 ford sport trac and had slight rough idle. After doing the usual MAF cleaning and other things and not solving my problem I watched Eric O's earlier vids about resetting the computer and that has fixed my problem both times.

    I have been guilty of regularly topping off my tank a few bucks at a time and also would sometimes run some E85 for the heck of it.

  43. Avataaar/Circle Created with python_avatars James Beemer says:

    Water in the gas from a bad refuel station can trigger that. Usually I just add octane booster to a full tank and it goes back to normal. Does not always work so I say just reset it like he recommends

  44. Avataaar/Circle Created with python_avatars Nate Who says:

    Oil can cause fuel trims to screw up??

  45. Avataaar/Circle Created with python_avatars Chad Gardee says:

    Got the blow module in there