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"Rumble" at light throttle only

Discussion in 'GM Powertrain' started by mateospeed, Jun 9, 2010.

  1. mateospeed

    mateospeed Member

    I have an odd problem, and would like some help brainstorming, as I've tried several things to no avail.

    Truck
    -1998 GMC Yukon
    -5.7L engine
    -4L60E tranny
    -stock everything

    Problem
    -At light throttle with the torque converter locked, in 3rd or 4th gear, the truck with "rumble" lightly. It comes and goes randomly, will rumble for about a second, then go away, then come back, and go away for a few minutes. At no throttle or heavier throttle, the rumble will not show up. It appears to be about 50-60mph, going up a slight hill is perfect to make it happen every time. Slower than 40 and the TC won't lock up. Coasting at any speed causes no rumble.
    -It is not related to speed. It will not happen in andy gear if the torque converter isn't locked up. It's possible that is actually is happening, but I can't feel it, as the un-locked TC would dampen the vibration.
    -It's a low-frequency (Bass) rumble, and not very loud. If the windows are down or the music is on, you can't hear it, but if you're paying attention, you can feel it.
    -I can't tell if it's coming from the front, center or rear of the vehicle.
    -NOTE: Air Conditioning was noted to not be working right after performing latest service. Not sure if it was working before, as it just got hot enough outside to need it.
    -When this rumble happens, the truck DOES NOT lose power (at least, if it does, I can't feel it). Engine speed stays constant.

    Work Performed
    -Replaced U-joints
    -Replaced rear wheel bearings
    -Replaced torque converter. solved another problem where the TC would not lock up, but did not fix rumble.
    -Used mechanic's stethescope to listen to differential and transmission for differential carrier bearing failure, heard nothing out of the ordinary.
    -Checked front driveshaft u-joints. They appear good.
    -Checked front wheel bearings by shaking un-loaded wheel. They appear good.
    -Replaced distributor cap and rotor
    -Replaced intake manifold gasket (un-related service, did not affect). NOTE: when I did this job, i managed to damage a spark plug boot, causing a misfire that was later corrected. I can't hear any misfire (arcing) now.
    -Cleaned upper intake manifold when replacing gaskets (coked with oil residue)
    -Replaced Power Steering Pump (leaking)
    -Changed Tranny fluid and filter

    So, I've done a lot of things. None of them have worked, which is a bit frustrating. If anyone has any ideas, I'm all ears! I have a few other ideas, but am unsure how to diagnose them:

    Possible Problems:
    -Engine Mis-fire: not sure why only at certain throttle, could be computer issue
    -Tranny issue: don't understand auto trannies very well, not sure what to pinpoint
    -Torque converter lockup problem: perhaps the actuator for the lockup clutch is pulsing, causing this feeling?
    -differential carrier bearings going bad
    -front axle shaft going bad(CV joints)
    -front U-joints going bad
    -Front wheel Bearings going bad
    -Front Differential Carrier bearings going bad

    Any other ideas? Thanks in advance for the help!

    -Matt
  2. wis bang

    wis bang Rockstar 100 Posts

    Maybe some spark knock...run a thak of premium and see if it disapears.
  3. mateospeed

    mateospeed Member

    Good idea! I'll certainly try that.

    UPDATE:
    I spoke to Monster Transmission, who sold me the torque converter, and gave me some very helpful advice in determining that my original TC was bad. They suspect there is an internal transmission issue, due to overheating (thanks to the bad torque converter). I'm inclined to take their advice with a grain of salt, however, since their solution is to replace or rebuild the tranny, and they sell rebuilt trannies. Thoughts?

    -Matt
  4. wis bang

    wis bang Rockstar 100 Posts

    Get a 2nd opinion, takeing any decient mechanic for a ride would reveal if it is spark knock or something more sinister. Your description sounds like spark knock and something going bad in the trans wouldn't get better when you stepped on it!

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