Vibration problem...rear diff?
 

Hi Folks,

Need some advice on a vibration problem. Sorry for the gory detail, but I figured I'd cut out a lot of the questions I typically see bounce back and forth in these forums.

I recently purchased a 1990 300ZX NA with about 70k from a private seller who basically kept it in his pole barn most of the time. The car is in really great condition (minus the problem herein). I put a brand new set of Yokohama ADVAN S.4 tires on stock rims shortly after purchase.

The car had a definite butt massaging vibration coming from the middle to rear end (I knew this going into the purchase, but I've waited 15 years to buy a Z and I couldn't pass this one up!). I have a good mechanic I trust and with his help and the help of this forum we thought had diagnosed the problem to the drive shaft/center bearing commonly discussed on the message boards. I found the run-out spec online, my mechanic measured the drive shaft and it was definitely out. I heard a lot of good things about a single piece aluminum drive shaft so I got one installed.

This made a definite improvement, but only seemed to shift the problem up to higher speeds. Before, it was annoyingly noticeable at 45-50mph, now it does fine until I hit about 60-65mph; above 65mph it continues to get worse... very annoying for highway driving.

So we have since investigated the rear diff and the right side has a bit of play (not a lot) if you push hard on the rear axle near the connection to the rear diff. The left side is tight with no play with the same test.

I've been unable to find a new rear diff online; I tried courtesyparts.com but they only have the TT version in stock; these parts have been discontinued by Nissan. I also don't like the idea of buying a used diff; I plan on having this car a very long time so any parts I install I want to know will last.

So I think my options are as follows and I'm looking for opinions on which way to go...

1. Rebuild current rear diff... my mechanic 'thinks' he can find a shop to send my rear diff to have it rebuilt.

2. Install a NISMO LSD part 38420-RS521.

3. Install a Quaife ATB LSD

I'm not really into racing or drifting; I love the power and acceleration the Z has and I do my share of speeding, but I really just want a smooth ride at all speeds. Sometimes I think the sound at high speeds could also be exhaust, but again I'm not sure of what's a 'normal' sound in this car.

Appreciate any advice one can give to this newbie Z owner.

Thanks,

John

I'd bet it's because you went with a 1 piece DS instead of a proper 2 piece. It's not common for the rear end to go bad, and it doesn't sound like yours is all that bad.

my car has EXACT same problem mate zworld told me it was the rear propshaft, but they charge ridiculous prices and they rip you off so i ent getting it done there, but yeh does it rumble alot within the first say half hour of driving, then it starts to get better?? mine does, if i start it in the morning and drive down the highway like you said yeh you can feel it alot but after bout half hour it dies right down and you can hardly feel it?

I have a short commute to work, so I've never had it at speed for that long since I've owned it. Right now I'm considering having them change the fluid in the rear diff to see if that helps and if it doesn't I'm thinking of going for the Quaife ATB limited slip differential.

Cheers!

John

Hate to ask
 

but are you sure you don't just have a bouncy tire cuased by a blown rear shock? Your describing a vibration that is affected by speed. Sounds like it could be a suspension part. Check your rear shocks. Have you ran the car while on a frame lift to see if you still get vibration?