4.3 acceleration after 55 mph

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by matteeeus, Jul 29, 2012.

  1. matteeeus

    matteeeus New Member

    My 2003 4.3 ext cab (100k miles) has issues accelerating after 55 mph.
    if i am completely stopped and i floor it it takes off no problem--Sluggish but its bc the 4.3, no surprise there.

    Scenario: *half tank, no load. 285 AT tires, with leveling kit. ALso, k&n clean air filter*
    i go on the highway, accelerate slowly to get up to max speed and lets say i have to floor it in order to beat a semi due to a merging lane.

    after i floor it my tech STAYS at 2.5 rpm and accelerates as if i am bearly stepping on it. Causing a freakishly slow acceleration that almost got me killed :mad:
    why is this happening? why arent my revs jumping to 5 or 6 rpm and causing a gas guzzling acceleration?

    Even though i usually always keep her under 60, there are situations like these were i have to hit 70 or 80 for a few yards. Any ideas?
    i do realize she isnt a v8 -__-
  2. JnBama

    JnBama Rockstar 4 Years 500 Posts

    Maybe your tire size , tall tires with the gearing in transmission and rear end. Is your speedometer off by a few mphs?
  3. matteeeus

    matteeeus New Member

    I just saw my typo, sorry
    they are 265's*

    my speedometer might be off a bit but it i have a sense of how fast i usually travel. My main issue being, why is it when i floor it--i get a wimpy response?
  4. JnBama

    JnBama Rockstar 4 Years 500 Posts

    best thing to is find a mech. for a diagnostic check
  5. billnorman

    billnorman Rockstar 100 Posts

    Sounds like your transmission is not downshifting.
  6. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    What is the factory tire size? If you altered the tire size, the transmission will not shift properly (depending on the amount of the size difference between the factory tires and your new ones).

    First things first... Get the speedometer re-calibrated. If there is still a problem, something else is wrong.
  7. matteeeus

    matteeeus New Member

    stock size is 235/16

    im at 265/16. : speedometer is 5.933% too slow according to an online calculator.

    what could cause my tranny to not downshift? there have been occurances were i accelerate to about 35 mph or 45 and the truck stays at that same speed/rpm for a few blocks as if i hit a cruise control. Clearly not normal. Im still in high school--broke. Id love to figure this out w/o taking it to a mech
  8. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    In 2003, GM started using a drive by wire system. You press the accelerator, the computer sees what you did, looks at several variables such as speed, transmission pressure, engine speed and load and then makes the decision as to what action to take with the throttle, fuel mixture and shifting. If any of those variables do not look good to the computer, it can decide not to accelerate or downshift based on those parameters and can opt to protect the driveline by simply doing nothing. There is no throttle cable. It is all controlled by the computer through solenoids and variable resistors. If for whatever reason it decides not to go, you can stomp it to the floor and it simply ignore the input from the throttle and nothing happens. It knows what the engine speed is supposed to be at any given road speed, and since it is not getting the input it is looking for, it might interpret it as a transmission slipping and ignore the pedal press. Hit it with a scan tool... One that reads transmission codes.

    Having said that, I doubt that a 5% change in speedometer reading is entirely to blame here, although, if the computer is too confused, it is probably in a limp mode, which will allow it to be driven well enough to get it to a shop... Safe mode, so to speak. I am guessing that it is a computer issue, however. This is likely not something you are going to correct in your driveway without some computer diagnostics.

    Please keep us posted.
    Last edited: Jul 30, 2012

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