Vibration on deceleration

[SteveJ]

I'm sorting out the problems on the 74 260Z I bought last month. Here is one that I need some more help in coming up with a definitive diagnosis.

This car has a stock 4 speed and (I assume) a stock differential with stock rear end gears. There are no outward signs that either have ever been replaced.

On accelerating through the gears, if I shift from 3rd to 4th around 50 to 55, I get a strong vibration, almost like the car is going to shake apart. If I shift early, say around 40 mph, and accelerate smoothly, the vibration does not appear until I let off the gas some. I could also get the vibration to appear at 35 mph when I let off the gas to prepare to turn.

I have looked at the trouble-shooting suggestions in a 73 FSM, a Clymers manual, and a Haynes manual, as well as searching online. Aside from changing the differential gear oil (which I did), it almost sounds like a parts swapping festival until you find the culprit. I'd rather avoid that.

I'd appreciate any suggestions to narrow down the trouble area.

Thanks!

[Arne]

First guess - U-joints. Check for play there, probably in the driveshaft u-joints, less likely to be the half-shaft joints. If you find any play in any of them, I'd replace all six.

Edit - I did not mean to imply that the half-shaft u-joints are less likely to be bad, but that they may be less likely to cause the type of vibration you describe. I would recommend that you check all six.

[SteveJ]

Thanks, Arne. It's one of those things that I'm not going to figure out myself, but as soon as you said to check the driveshaft, I realized how that could be the culprit.