99 burban wont shift under hard acceleration

Discussion in 'GM Powertrain' started by Stpndwn, Jun 26, 2011.

  1. Stpndwn

    Stpndwn New Member

    Hey guys, Im beating my head against the wall... 99 burban Z71 shifts fine unless you are in the throttle. no codes are set or stored and the timing is correct. ckp and cmp both check out. No trans codes are on ether. when under hard acceleration you have to let up so it will shift to next gear. No it does not shift hard. Fuel pressure is 55psi at idle and bleeds down to 40 in about five min after shutting it off. Changed the fuel filter and still has same pressure readings. Does the low pressure have any thing to do with the shifting issue? Where do i go from here? Please help.
     
  2. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Sounds like you need to take a close look at the throttle position sensor. There is a way to test it, and that is what controls how the tranny shifts, and if there is a dead spot in the TPS then you will have issues like you are describing.

    As to the low pressure. you are supposed to have 55-65 PSI with the key on motor off. And since your motor is starting you have more than 58PSI key on. As ot it bleeding down there is a good chance that you need to replace the Fuel Pressure Regulator since it is pretty common for them to leak down with age.
     
  3. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    What happens if you just stay on the gas?
     
  4. Stpndwn

    Stpndwn New Member

    Hangs out around 3600 rpm and wont shift. i do believe its the TPS. going to test it rite now. ill update in a while. Thanks for the insight, Mike.
     
  5. Stpndwn

    Stpndwn New Member

    So it was not the tps. It is still hanging out around 35 to 4500 rpms and won't shift unless I let up. Beginning to think its a selenoid in the trans the is hanging up. It does this in every gear.
     
  6. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    I would take it to a shop with a Tech II or equivalent scan tool.
     
  7. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    X2 on that since it might not be throwing any codes but a tech II can pull tranny codes and that will give you alot more insight!
     

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