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Your 4.8 4x4 fuel economy..

Discussion in 'General Chevy & GM Tech Questions' started by thunder57, Dec 10, 2012.

  1. thunder57

    thunder57 New Member

    Hello all,

    I am new to the site but have used it in the past to help modify my truck. The truck is a 2002 regular cab 310 000kms - 192 000 miles, - step side. 4.8 auto, z71, 3:73 gears. I absolutely love the truck and it's been great to me so far.

    I just did a lot of highway driving over the last two days with little city driving. I have recently replaced the plugs and wires, fuel filter and it has a ebay air intake kit.

    I got an amazing 11.8mpg while doing driving between 60 and 70 mph over a couple long trips. Seems a little low to me..
    If anyone else could chime in with their numbers that would be great.

    I have tested the fuel pressure and it tested pretty low.. 41 psi, it does have one engine code for the evap filter being clogged.

    Thanks for any help.
  2. Pikey

    Pikey Moderator Staff Member 2 Years ROTM Winner 1000 Posts

    Have you changed out the fuel filter? I am not saying that that is causing your low MPG but, you did mention changing plugs and wires. That does seem pretty low. My 2005 yukon xl 4x4 5.3L gets between 18-21 mpg on the highway depending on conditions. My 02 silverado extended cab 2wd 5.3L consistently got 24 mpg cruising at 72mph, with a jet programer, it had 3:43 gears. What kind of plugs did you put in? I got talked into some Autolite plugs for the 2002 one time. It dropped my mpg significantly. I noticed this after one 800 mile road trip. I changed to plugs out to the AC delco that GM recommended and the next week when I did the exact same trip my mpg went back to normal. I have never had a 4.8L but those numbers seem pretty low. What pressure are you running your tires at? I know with the yukon if I run 32 psi I get around 16mpg if I run 35psi I get much better numbers
  3. thunder57

    thunder57 New Member

    I am running NGK TR55 3951 V-Power Spark Plugs and I replaced the fuel filter with the plugs. My tires are 265/75/16, so a little bigger then the stock ones, filled to 50 psi. Would low fuel pressure cause poor fuel economy? Thanks for the help.

    Jake

  4. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    For some reason, the 4.8s appear to offer poorer (by 1, or 2 mpg) mileage than the slightly larger 5.3. You should be seeing around 17 to 18 driving at 65 (IMO).

    I like to change the front O2 sensors when I change plugs. The sensors will get tired and switch slower (may still be in spec, just slower).

    Tires a little bit bigger should hurt, the air pressure is good, but what thread pattern are they? The best mileage is with an almost smooth pattern, like a touring tire, the more aggressive the thread pattern, the higher the rolling resistance.
  5. thunder57

    thunder57 New Member

    Here's an update..

    I put in a new AC Delco fuel pressure regulator, and a new AC Delco fuel pump/sending unit.

    My pressure with key on is 50 and running it is 48.
    With the vacuum line off the FPR pressure goes up to 58
    With the vacuum line on and return line blocked I get 58

    Any input would be great, thanks!
  6. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    Put the factory spec plugs back in it, vortec motors just don't run we'll on anything but the ad/delco plugs, if you do a search you can find many threads on here that agree with that. I have 2 trucks with different motors from different line of motors and the same is true for both they ran like crap on anything but ac/delco iridium plugs.....
  7. thunder57

    thunder57 New Member

    Just wondering what AC Delco plugs I should get. Thanks



  8. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    Any parts store will be able to tell you the exact number, they have changed over the years as they used to be called double platinum but now more common as iridium tip you just want to make sure you have OE plugs in it. Also check your fan shroud under the hood, they usually will show a part number on it as well, right next to the picture of the correct flow of the serpentine belt

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