4.8 don't feel right

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by newfiebullet, Sep 8, 2013.

  1. newfiebullet

    newfiebullet Member

    had my truck 3 years now and have 65000 kms on it. for the first 6 months all seemed well. but since that the power just isnt there, 3000+ rpm to hold speed on the slightest hill. last weekend i towed my atv out over the hiway , probably 1200 lbs counting the trailer, and could barely manage 11 mpg, 4500 rpm on hills to maintain speed and very slowly pickup. iv tried a few things but no sucess, at one point had a cold air intake, mbrp exhaust and a diablosport programmer but could feel no noticeable difference over a couple months so i sold the intake and programmer but kept the exhaust cause i like the sound.is anyone elses 4.8 like this? dealership has hooked up it many times and say everything is right.near 300hp shouldn't struggle like this. any help or suggestions? truck is a 2010 silverado 4.8 with 3.42 gears , mbrp exhaust and 285/70/17's on a 2" level. tires and level have been on since day one and any helpwould be great.
  2. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Sounds like you have lots of horse power (according to dealer), but it is being used for something other than moving the truck at a "normal" level.

    I'm thinking there is more than normal resistance to move the truck forward. Something like dragging brakes, flat tires, out of align tires, etc.

    If you take the truck to a hill and release the parking brake and place the transmission into "N" and lot it roll down the hill, does it seem to roll easy, or is it slow to move?
  3. newfiebullet

    newfiebullet Member

    no resistance, she coasts down hills free as a bird, surprisingly well actually
  4. j cat

    j cat Epic Member 5+ Years 1000 Posts

    with what your saying I would check the fuel pressures then I would have the exhaust checked for restrictions. these failures will not show failure codes until it gets more advanced.

    no codes/low power/low MPG issues...
  5. newfiebullet

    newfiebullet Member

    not sure on fuel, never had a code and the exhaust is a free flowing mbrp system, no guts in the muffler...something isnt right and shes harder on gas then ever, on a good day on the highway im lucky to see 17 - 17.5 mpg
  6. j cat

    j cat Epic Member 5+ Years 1000 Posts

    so you have no cat converter then . well that would rule out the exhaust. this means that the fuel pressure may be off too low or your MAF is with some oil on it . many with the cold air intakes need to spray these small sensor wires often with the CRC MAF cleaner , just make sure you do not touch those wires though.
  7. newfiebullet

    newfiebullet Member

    i have cats, just ment the muffler wasnt plugged, cats shouldnt be plugged this early? will try the maf clean tho
  8. stchman

    stchman Active Member 2 Years 1000 Posts

    Let me ask a question:

    Did the power loss appear when you put this larger tires on? The stock tire height on that truck was ~3" less in diameter and larger diameter tires will rob you of low end torque.

    Also does the ECU know the new tire height? If not you need to get another tuner to tell the ECU the height. When your ECU thinks you have smaller diameter tires than you actually have, erratic behavior is what you can get.
  9. newfiebullet

    newfiebullet Member

    tires been on since frst week with the truck, my stocks were 265s..i only went to 285's. like 1.25" differnce.had the programmer and it did nothing, and i mean nothing no power increase, no torque managment reduction , no increase in mpg nothing. diablo checked it out for me a few times and said everything was fine with the tuner. im lost , this is pretty crappy , ill try the maf cleaner and get an ecu reflash when i get my next oil change. cant hurt for sure.
  10. j cat

    j cat Epic Member 5+ Years 1000 Posts

    if you have a problem with fuel delivery/oil consumption / or something is not properly metering the fuel these converters can plug up fast. normal usage with no failures they can last the life of the vehicle as long as they do not have to burn off excessive fuel. THEN THEY PLUG UP. this is why you do as I recommended so that cause is done . period. then you also check fuel pressures. make sure the pressure is correct at engine off key on and after you start engine .

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