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Thread: Experience with Horrible Gas Mileage on a GM 5.3L Z71?

  1. #1
    Join Date
    May 2011
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    Default Experience with Horrible Gas Mileage on a GM 5.3L Z71?

    I am having Fuel Consumption Issues with my '04 GMC Sierra 5.3L Z71. When I say Issues, I mean "11mpg" coming or going. The truck has 118k on it, and seems to run well. I purchased it wrecked last year and repaired it. Personally, I've put less than 1k on it. I've done the preventative maintenance on it and gone through the fuel system... still drinks gas like its attached to the Gas Pump.

    I have replaced:
    Fuel Pump with an O.E. Unit
    Spark Plugs & Wires with O.E. Parts
    Air Filter with K&N Factory Replacement Size
    Air Mass Sensor with AC Delco Reman'd Unit
    3" Exhaust from Cat. Conv'rs to Tail Pipe

    My next step would be to Re-flash the ECM, and possibly do an Upgrade Program. Has anyone encountered this type of an issue on a Silverado, Sierra, Tahoe, Yukon, or Suburban?
    NATHAN HACKETT
    '02 CBR954RR

  2. #2
    Join Date
    Feb 2010
    Location
    Fort Worth
    Posts
    1,622

    Default

    Is that truck Geared? It looked a little lifted, that will kill mileage I am sure you know.

    What are those trucks suppose to get one stock?

    Only thing I could think of if the parts don't go with the ecu and are causing it to run rich?
    "If you have to sign a wavier, it is probably worth doing."

    CMRA #728

    Brandon
    2006 GSXR600 Trackbike
    2007 Aprilia SR50
    1964 Vespa ET3 motorswap

  3. #3
    Join Date
    Sep 2005
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    12,299

    Default

    1) Replace the first/front 02 sensor (the one b4 the cat).
    2) get a tune from http://www.blackbearperformance.com/
    Michael
    445whp LS3 Miata

  4. #4
    Join Date
    May 2011
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    Default

    Brandon, the Truck is lifted a few inches, and has stock gears. Average Mileage is 15/19, which is fairly consistant with every owner I have spoken to.

    ZedX, what is the reasoning for replacing the O2 Sensor? When I watch the Scanner's Live Readout, all 4 of the Sensors are functioning properly.
    NATHAN HACKETT
    '02 CBR954RR

  5. #5
    Join Date
    May 2008
    Location
    Aledo
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    You can also try replacing the temperature sensor on the intake manifold. They are notorious for false readings causing the truck to run rich. Make sure there are no vacuum leaks.
    Last edited by CBar; January 17th, 2013 at 06:16 PM.
    Chris
    2008 Buell XB12X Ulysses
    2004 KTM 450 MXC
    1996 Bimota SB6
    1988 Honda Hurricane 1000
    1974 Norton Commando 850

    Email tsba.ftw@gmail.com to inquire about the Fort Worth Chapter of TSBA.
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  6. #6
    Join Date
    Sep 2005
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    Default

    Quote Originally Posted by NathanNTX View Post
    ZedX, what is the reasoning for replacing the O2 Sensor? When I watch the Scanner's Live Readout, all 4 of the Sensors are functioning properly.
    They usually are due being changed as you approach 100k miles. The environment they live in isn't exactly sensor friendly.

    Do a Google search, but this is just the first hit on the one I did for you... http://www.mpgenhance.com/oxygensensor.html
    Michael
    445whp LS3 Miata

  7. #7
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    Interesting Recommendations... I will definately check into them both. Thanks!
    NATHAN HACKETT
    '02 CBR954RR

  8. #8
    Join Date
    Dec 2002
    Location
    Sugar Land, TX
    Posts
    731

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    Have you tried cleaning the fuel and air intake systems?
    Last edited by Tim Vipond; January 19th, 2013 at 05:57 AM.
    Tim Vipond
    2006 K1200LT with 70,000 AMSOIL (changed yearly) road miles, 3 AMSOIL MSRH laps
    AMSOIL Dealer
    http://www.lubedealer.com/TimVipond/ cell 713-301-1872 25% discounts available.

  9. #9
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    May 2008
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    Aledo
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    How is the idle after the truck has warmed up?
    Chris
    2008 Buell XB12X Ulysses
    2004 KTM 450 MXC
    1996 Bimota SB6
    1988 Honda Hurricane 1000
    1974 Norton Commando 850

    Email tsba.ftw@gmail.com to inquire about the Fort Worth Chapter of TSBA.
    https://sites.google.com/site/tsbaftw/Home

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