Hard vibration on deceleration

[srj67]

I recently purchased a 1976 280z to restore for my 16 year old daughter (my first car was a 1977 280z). Driving it home I noticed that if you are above 60 MPH and let off the throttle it starts to vibrate, and if you let off at 80 MPH it tries to shake the hood off. I figured a bad u-joint, but I checked the main shaft and half-shafts and everything looks tight. Could it be a bad differential strap/mount, or is there something else that someone has ran across?

[LostxSoul]

It does sound like a driveline issue but in reality if I remember correctly it should make noise under load or acceleration if there's an issue rather than after you let off the throttle. Now for clarification... "looks tight"? You shouldn't have much if any "slop" in the driveline or half shafts. Give them a good twist and see if they rotate much back and forth. You can unbolt the driveline and turn it 180 degrees and that rotates the driveline around so the weights are on the right side for balance. If someone took the driveline off (previous owner) they may have forgotten to mark it and just threw it back in with the weights on the opposite side.

Does it vibrate from the front (feel it in the steering wheel) or the rear (feel it in your seat)? If you just bought it, it might not be a bad idea to get her up in the air and give it a good once over.

I've had to pull my driveline twice in the last couple months due to a leaky output shaft seal. It was on a lift and still kind of a pain. Unless this is upgraded most of the drivelines on Z's out there are staked drivelines. Meaning you can't service the U-Joint. You can look around for an upgraded one with servicable U-Joints but it's best to find the real issue first before throwing money at it. I think they run about $250 or so.

Give it a little bit, someone more knowledgeable than I will probably give you more ideas and other options to look at.

Edited April 17, 2011 by LostxSoul

[Gary in NJ]

It's the axle shaft joints (CV joints).

[doradox]

It's the axle shaft joints (CV joints).

CV joints? I wasn't aware that a 280 Z had those. I thought they were Cardan joints.

Steve

[srj67]

Definitely no CV joints, just regular u-joints and driveshafts. It appears the half shafts have been serviced, as the u-joints actually have grease fittngs in them (although in 1976 they may have actually had them from the factory, I'm not sure). There is no slop in any of the three drivelines or rear suspension. I have not given too much attention to the differential yet, and I need to get the front end in the air to do a more thorough inspection. I agree with LostxSoul, it really feels like a driveline issue. I lost a u-joint one time in a truck, and it would vibrate when you unloaded the driveline for that time period between letting off the throttle and actually going on compression.

[Zforce]

Is it a manual ? Could be the clutch slipping, what happens when you step on the clutch pedal ?

[LostxSoul]

Get it back up to speed, say 65mph when the issue normally happens then drop it into neutral and see if it still makes the vibration issue. This should take the load off of the driveline for the most part. Although it'll still spin even out of gear there is a load on it when you back off the throttle in gear. When you take it out of gear if you still get the vibration, your DL might be out of balance. I don't think it's that expensive to get it balanced properly, plus a good driveline shop can tell you if it's out of balance in the first place.

Differential issues usually show up as humming noises (gear issues). I think if the mustache bar and diff strap were bad you'd get a bit more issues as you accelerate and run through the gears but I could be mistaken.

[IdahoKidd]

I'm jumping on the driveline bus too. Ujoint or balance issue. (Bent would be a balance issue) Could be a messed up diff, had that event once in a different car, but it revealed itself about the 4th time I let off the throttle to try to find it. Drive line came out still attached to the pinion flange. Could e a lot of things, even including a separating tire. Let us know what you find. I see you are new. Welcome!

[srj67]

Thank you all for the replies, and thank you for the welcome. My first car was a 1977 280z, and now I am restoring a '76 for my 16 year old daughter. To answer one question, it is an automatic - for now. I will get it up in the air a bit further and give everything a good once over. I will let you know.