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456 shudders under light acceleration.

Discussion in '456/550/575' started by skl63, Apr 13, 2010.

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  1. skl63

    skl63 Karting

    Dec 13, 2007
    228
    Melbourne, Australia
    Full Name:
    Scott
    #1 skl63, Apr 13, 2010
    '97 456 GTA. Shudders under acceleration in top gear at about 80-90 kmh. Has done this for as long as we have owned car. Can be solved by manually dropping back to 3rd, or kicking down to 3rd.

    My diagnosis says it is because the torque converter is bi-passed in top gear and engine is lugging at about 1800-2000 RPM.

    Has anyone else experienced this issue, which is the only significant fault with a great car.

    How cleanly does a manual pull in top at about 2000 RPM?

    Is there any adjustment for the shift points. i.e, is the car shifting into top gear to early?

    Thanks
    Scott
     
  2. tajaro

    tajaro Formula Junior

    Mar 22, 2009
    686
    Gulfport Florida
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    Erik V
    #2 tajaro, Apr 13, 2010
    My manual pulls without hesitation or shudder from 1000rpm in 6th. With the amount of torque these cars make it shouldn't be engine load that makes it shudder I don't think. Perhaps your diagnosis on the TC is right on- it might be in the range of the stall speed or something... Good luck.
     
  3. oss117

    oss117 F1 Rookie

    Jan 26, 2006
    4,185
    Plantation, Florida
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    Alfredo
    #3 oss117, Apr 13, 2010
    Last edited: Apr 13, 2010
    If I had to guess, I would say it is electrical and not mechanical.
    I would check all the electrical connections under the hood and in particular the battery ground.
    Check all the plug wires and unless you have done that recently, replace the plugs.
    How does the car respond after it goes past the critical rpm where it shudders?
    Do you get full power in top gear with the surge after 4,000 rpm?
    To give you an idea, I can go as low as 1,000 rpm in 6th and then accelerate without any problem.
    The acceleration is not brisk at first, but it is no slouch either and there are no hesitations throughout the rpm range.
    Good luck.
     
  4. skl63

    skl63 Karting

    Dec 13, 2007
    228
    Melbourne, Australia
    Full Name:
    Scott
    #4 skl63, Apr 13, 2010
    The car pulls strongly once struggling through the "shudders". Revs out to the Red and generally runs beautifully.

    It makes the car quite unpleasant at our normal "highway in light traffic" speeds.

    Electrical fault? Why would this be expressed at these low speeds?
     
  5. Dave 456

    Dave 456 Formula 3

    Nov 15, 2007
    1,317
    Sydney, Australia
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    Dave Simons
    #5 Dave 456, Apr 13, 2010
    Silky smooth and like a train..... However, it had a couple of vibration periods before I changed the engine mounts - one bolt head was touching the chassis sometimes.

    Getting the injectors overhauled (peanuts cost) by a Bosch service shop made a difference to just about everything - consumption, throttle response, smoothness. Best couple of hundred dollars I've ever spent.
     
  6. oss117

    oss117 F1 Rookie

    Jan 26, 2006
    4,185
    Plantation, Florida
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    Alfredo
    #6 oss117, Apr 14, 2010
    When the engine runs at low rpm but the transmission is in high gear, you are demanding the most torque.
    Under these conditions the engine tilts the most on its mounts.
    If you have an imperfect electrical connection the contact is going to be lost, then regained, lost, regained and so on, as the engine tilts back and forth.
    That is when you need a strong and steady spark from the plugs and good atomization from the injectors.
    It would be good also to check the engine mounts, just in case.
     
  7. skl63

    skl63 Karting

    Dec 13, 2007
    228
    Melbourne, Australia
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    Scott
    #7 skl63, Apr 14, 2010
    If it is an ignition miss, as a result of poor earth or whatever, I guess I could probably confirm on a dyno?

    Tnx
    Scott
     
  8. PAP 348

    PAP 348 Ten Time F1 World Champ
    Lifetime Rossa Owner

    Dec 10, 2005
    100,216
    Mount Isa, Australia
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    Pap
    #8 PAP 348, Apr 15, 2010
    Yep, you should be able to mate. :):)

    Another easy test is to put the car in gear, handbrake on tight..........press the brake pedal hard and slowly bring the revs up while in gear. :):)

    This will load up the engine and you will soon see if there is an engine miss. Pop the bonnet and check if there is excessive engine movement while doing that. :):)

    I hope you understand what I am posting. Its easier showing you how to do it and check those mounts. :D:D
     
  9. skl63

    skl63 Karting

    Dec 13, 2007
    228
    Melbourne, Australia
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    Scott
    #9 skl63, Apr 15, 2010
    Thanks Pap, I understand exactly what you mean, although I suspect I would not get to the 1700 ish RPM. Strange that the issue is tied also to a locked out TC. Also, pretty confident that the car has new uprated 575 mounts, but I will check.

    Interestingly, in the last two days I have spoken to two of the designers of this transmission, both ex Ricardo. One lives in the UK and one in Australia. Additionally, the TCU was developed in Australia for Ferrari.

    Apparently there is also a small button battery in the TCU, probably not relevant and probably only used during the initial calibration process of the box.

    FWIW, both guys said the trannies are pretty robust and neither of them could suggest anything very concrete, even suggesting that is not likely the tranny at fault.

    Things you learn
     
  10. Dave 456

    Dave 456 Formula 3

    Nov 15, 2007
    1,317
    Sydney, Australia
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    Dave Simons
    #10 Dave 456, Apr 15, 2010
    Ah-Ha! So that's why the only 456 autobox failures have been in the northern hemisphere ....the electrons are going the wrong way round! It needs an energy polariser......
     
  11. PAP 348

    PAP 348 Ten Time F1 World Champ
    Lifetime Rossa Owner

    Dec 10, 2005
    100,216
    Mount Isa, Australia
    Full Name:
    Pap
    #11 PAP 348, Apr 15, 2010
    Ok man, good luck with it. :):)

    You learn something new everyday. :D:D
     
  12. oss117

    oss117 F1 Rookie

    Jan 26, 2006
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    Alfredo
    #12 oss117, Apr 15, 2010
    Hi, you may have your answer right there: the locked out TC.
    That could be the source of your on / off engine performance.
     
  13. mont

    mont Formula 3

    Sep 16, 2004
    1,551
    Portugal
    #13 mont, Apr 15, 2010
    Hi,
    I would check the sparkplugs. Champion or NGK? If you use the Champions just put them far, far, away...
    Very small vaccum tubes around the intake/plenum manifolds both right and left side... damaged gaskets on each intake port...

    Always look at the bright side, first.
     

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