Question about 1990 GMC C1500

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1989 gmc sierra c1500 with 350 automatic 8 ft bed regular cab vibration at 45 in overdrive

My 89 gmc c1500 truck(350 automatic) on a slight uphill grade while in overdive at 45-50mph sets up a vibration in the whole vehicle. If I manually shift it to 3rd gear the vibration goes away instantly. My inpection mechanic thinks it is a cracked flex plate(flywheel) but visibly no cracks can be seen from the bottom inspection pan all the way around. I also think my torque converter seal is leaking. If I do this job should I replace, new seal, new torque converter, and flex plate. How do I tell which transmission I have, the advanced auto guy said it could be one of two different kind. I looked at it from the bottom and could not find the markings he said.

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  • Anonymous Apr 08, 2009

    I have a 1998 gmc sierra c1500 with 350 automatic kingcab it shakes at 60 going up hill when in a pull.

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I had the exactly the same problem, in overdrive my truck would start to shake, in 3rd it would instantly go away, i assumed it was the flywheel to, THIS IS NOT THE PROBLEM, your tranny is in good shape and flywheel are in good shape, I had the "Idle Motor" replaced is what my mechanic called it, this completely fixed my problem, also my fuel milage went up considerably after having this replaced, hopefully this will help you out

Posted on Mar 11, 2009

  • xxxderek16 Mar 11, 2009

    It was only a $200 fix also***

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