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bucking between 55-70 mph

Discussion in 'GM Powertrain' started by dsquared, May 26, 2010.

  1. dsquared

    dsquared Member

    I have a 97' K1500 w/235K miles. I replaced my transmission about 18 months ago with a GM serta. Plugs, wires, cap & rotor button and fuel filter replaced recently. Engine idles smooth, accelerates like a new truck after I replaced the fuel filter(what a difference ! ). I have a "bucking" when I accelerate lighty at highway speeds between 55-70 mph. I dropped the tranny in 3rd at those speeds and when I accerated gently and I couldn't reproduce the "bucking". Is there something possibly wrong with the tranny in overdrive or is there another problem else where? The tranny shifts smoothy in every gear. There is a fault code for the 2nd gear solenoid so I took it to the dealer under warranty and they replaced the solenoid but the fault code stayed on. They said there is something wrong with the PCM, some kind of short. What say you.
  2. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Have you checked for any other codes? There is a chance that is being caused by the tranny, but another fault that I have run into with these trucks is the bushings in the distributor will go bad, causing a random cylinder misfire..
    Also when you are dropping it into third gear you are taking it out of OD into third you are raising the engine RPMs which will eliminate any "lugging" of the motor.. I am not saying it is either of these since it would be a good idea to pull the codes to give you a better starting point.
    I am sure that there will be others to join in and add in their expertise as well.. Good luck
  3. dsquared

    dsquared Member

    If it was a worn distributor bushing wouldn't it happen more frequently and at varying loads and speed ? I'll check for any other codes.
  4. Dr_Zero

    Dr_Zero Epic Member 5+ Years 500 Posts

    Sounds like a timing issue, the computer will try to adjust the spark but if its off when the computer adjust things it wont work right and you get the random misfire code also.

    When I pulled out the distro for the intake manifold I got off just a tad and mine did that moved it back a tooth and all was fine.
  5. ThatSilverStepside

    ThatSilverStepside Rockstar 4 Years 100 Posts

    Did you buy the trans used?.is it auto or manual? ..I had a similar problem with my 700r4..it lasted about a month after it started. Now its smoked..I flushed trans flud and changed filter and it stopped for a while then started again..could also be a problem with your computer causing the trans to misshift. Good luck
  6. dualdj1

    dualdj1 Rockstar 100 Posts

    have an issue on a lancer where it'll give a very light buck at constant throttle running at similar speeds down the road. friend told me it was a prob with the trans (not driveability threatening), and the comp picked it up as a knock and pulled timing. Could be something like that depending on how sensitive the knock sensor is.
  7. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    On one of the trucks that ive had to do it on recently it only acted up in OD... But yes usually i was just trying to give you a few things to consider so you at least have a few places to look depending on the codes. The first time i had trouble with a distro, it took 2 weeks of troubleshooting and ended up not fixing the problem till the cap actually split because the rotor hit it so hard..
  8. dsquared

    dsquared Member

    How much play in the distributor shaft would be too much ?
  9. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Best bet is to look at the cap, and see if the rotor is hitting any of the connectors harder than the others since usually it is a wobble, that wears funny.. And the play that you are looking for isnt actually the turning play, you are looking for an ability to move it front to back etc... IF you get what i mean?? Im trying to explain it and the words are escaping me as of this moment.
  10. dsquared

    dsquared Member

    I understand what you mean - radial play or side to side movement. I'll take a look at the cap. I'm still not sure why this would cause a misfire at these operating speeds and not accross the speed range and especially during hard acceleration.

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