Help! Vibration when accelerating?

[1960chevypanel]

Have an '84 C20 454/turbo 400 rear wheel drive pickup with 65K og miles. She just started doing a deep/low vibration when I accelerate over 35mph. If I take my foot off the gas the vibration stops. The trans has never given me any problems, the fluid is pink and full. I just checked the u-joints visually and don't see any obvious problems, and don't feel any excessive play. Has anyone else had this issue? I'd like to brainstorm up a list of 5 or 6 things that I can check. Need to get the truck back on the road asap.

[kdh454]

I had a similar issue, turned out that one torque converter bolt was missing and the other two were loose. May be worth checking. Otherwise, maybe you have a bad motor/transmission mount?

[tucsonjwt]

I would go to a local repair shop who has a $25 lube, tire rotation, and inspection. Tell them you have a vibration and ask them if you can look at the truck when it is up on the rack. You might get a diagnosis from that shop, then you can go to another shop for just a free inspection and see if they have the same diagnosis. If you have a tire problem the rotation may change the characteristics of the vibration. You can also have someone rev the engine on the parked truck while you look under the hood to see if the engine is moving left or right, indicating a bad motor or trans mount.

[1960chevypanel]

Thanks guys, I ended up doing a "driveway dyno test" at 35mph and could actually see the problem in motion. Check it out:

[foamypirate]

Holy smokes, that sucker was toast!

[RDrancher]

Well, at least you got your money's worth out of that one.

[wilkin250r]

Whenever a vibration problem appears only under acceleration, I always suspect the joints. Any rotating object on/in your car that is inherently off-balance but is still mounted properly will always vibrate, on or off the gas, it won't change with power delivery.

The U-joints are one of the very few items that have the potential to change shape under power delivery and CREATE a vibration. The other suspect that changes with power delivery is your differential, but it's usually a noise rather than a vibration.