Serious Vibration On load, Off load, coasting

TerraLR93

New Member
Feb 12, 2022
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Bethlehem
Hi all. I have a 1989 Mustang GT. It has developed a serious vibration but only under specific circumstances.

At highway speeds, 60+ mph. When I'm in gear, I place my foot on the gas pedal, the vibration happens for a moment and disappears. When I release the pedal, it does the same. If I'm in neutral, clutch in, or anything that seems to take the transmission out of the equation, it's constant, and pretty intense. I don't feel it in the steering wheel, but underneath of me, in the seat. I can hear it too.

In the past, I've had the driveshaft out for transmission issues, but I made sure it was tight. At least one broken u-joint was fixed as well during that time. I've also replaced the transmission crossmember within the last 4 years.

Is there anywhere based on these circumstances I should start my search?

Thanks!
 
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Check those ujoints again. Not just wiggle the shaft when installed but remove the driveshaft and rotate the ujoints by hand. They should be totally smooth in movement with no clicking or binding feeling.
 
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