Dash vibration around 60-90mph

[JayTA98]

Hi all. This my story. 1998 TA 56k A/T. I have the car for 6 months now. I had a light vibration on the rear of the car for a while. Nothing too serious or annoying. Today I took the car to my friend that is a mechanic. He told me to have the tires balanced which I did but the vibration is still there. I bought 2 new tires for the back and have them balanced. The vibration is still there. The shop told me that the rims are good. We checked the transmission mount. It was broke. As soon as we jacked the transmission a bit you can see that the rubber completely separates from the mount. I went and bought the mount but I got the polyutherane one since they are better. So I got home and lifted the car on one side. I put a second jack on the tranny to lift it up a bit I removed the crossmember and then the tranny mount. I followed the instructions to put the new mount. When I tried to put the crossmember back it was too far from the metal for the bolts to reach. I lifted the tranny jack maybe an inch or two and it aligned perfect. I tighten all the bolts down as hard as I could and I lower the jacks.

I went for a test drive and the vibration was still there but it was stronger this time. The problem started when I go to the highway. When I reach 60 mph it starts to vibrate on the back and the center of the car. No vibration on the steering wheel at all. I can feel the vibration on the shifter but lightly. If I hit the gas it downhifts to 2nd gear and when I get around 70 to 80 mph the whole dash starts to vibrate really hard that I can hear it. The vibration on the back stays the same. This dash vibration was not happening with the old mount.

Could my U joints be bad? I checked them and they don't move at all. Any pointers? Thanks in advance.

[Tommycat]

Sounds like a possibly unbalanced driveshaft. Though it could be one of your rear disks is warped.

[0rion]

first off the poly mount is going to transfer more vibration into the car so that's why it got worse and probably why you hear it in the dash now. When you hit the brakes do you feel anything in the pedal? A lot of times if a rear rotor is warped that's where you'll feel it. The pedal will pulse. Any way you could've bent an axle or something like that? Do donuts in the car at all? Probably not the culprit but just ruling it out. My personal experience has been that high speed vibrations like that are usually tire,wheel, or brake related. Not always but 90% of the time. You've ruled out tires and wheels right? If you wanted to be positive just swap the rears up front. Brakes are a little tougher to rule out. You could take them somewhere and have them turned and they'll be able to tell you then if they're warped. If all of that checks out I would start looking at the u-joints, driveshaft, and rear end next. It can be a pain in the butt chasing this stuff down and it'll get expensive if you just throw parts at it. I wouldn't replace anything until you've ruled out all the easy and free to check stuff.

[Street Fighter]

I am dealing with the same problem with my 77 camaro,and have done all the things you have done and still have a vibration from 80 and up that's really bad.I can hear and feel the vibration. when you find the problem let me know and if I fine it I will post it so you will know. I have spent alot of money trying to find the problem and haven't yet.I have worked down to the rearend.I have gone as far as changing axles and bearing,still no luck.I feel now that it's in my rearend, but I hate to go that route and spend all that money and that's not it.I say that it may be in the rearend because when you let off the gas you can feel the slack moving in and out on the rearend.

[JayTA98]

Ok this is where I'm at so far. Since the tranny mount was the only thing that we changed I decided to buy the GM tranny mount($112, those bastards). We checked the U joints in the drive shaft. We moved the driveshaft and it didn't had any signs of the u joints being bad. I had previous experiences that they are bad but you can't see it until you pull the driveshaft out and then check them. We pulled the driveshaft and the u joint on the rearend was bad. We replaced both u joints(front and rear) and retest the car. The dash vibration was still present but the rear was a lot less. I then swapped the polyurethane mount with the OEM mount from GM. I took it for a test drive on the highway running between 70-130 mph downshifting and accelerating hard and the vibration in the dash is completely gone. I still have a small vibration that feels that is from the back but is 90% less than before. I'm going to replace the 2 front tires since they are old and rotate them to the back and see if I maybe have a bent wheel. Thanks for the help.