I have a odd vibration porblem that I can't seem to fix. Here is the story:

1985 Fiero, 4 cylinder, 5 speed. I have a vibration in apprently the rear end as I am feeling it in my butt.

So far I have eliminated tires and wheels through a series of things - had tires balanced, didn't fix it, checked that wheels were true (they are), even changed out to another set of wheels and tires I had and problem still exists.

I had a new clutch installed with flywheel so that is good. Plus the vibration is still there if I knock it out of gear at 50 MPH and even if I turn the engine off, so it isn't related to transmission or clutch.

It is the worst between 40-60 MPH and is almost gone at 65MPH (you have to think about it to notice it at that speed). Doesn't seemt o be affected by engine RPM or what gear I am in (4th vs. 5th). Almost feels as if a tire is out of round but as I mentioned, I had all of that checked as it was my first thought.

Any thoughts/suggestons?

Thanks

Robby

Grab the axles just outside the transmission housing and yank them back and forth.....you may have a bad bearing.

It does sound like a wheel bearing. BTW: vibrations do go away at higher RPMs; they just get closer together and thus are less noticable, which can make one think all is fine!
Onoe more question, does it become less noticable when you back off in gear?

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quote Originally posted by Francis T: It does sound like a wheel bearing. BTW: vibrations do go away at higher RPMs; they just get closer together and thus are less noticable, which can make one think all is fine! Onoe more question, does it become less noticable when you back off in gear?

No I haven't noticed any change when backing off a gear. I was thinking axle bearing but if it were to be a wheel bearing it would be much easier to replace.

forgot to mention...when checking the axle for movement make sure the car is on it's wheels.....if you jack it up everything seems to tighten up and you think it's all fine, but it isn't.

Check your axle nuts! If one is loose, it will cause a vibration - eventually will destroy the wheel bearing. The axle stub/nut holds the rear wheel bearing together. Even if the axle nut is tight, it coud be a bad bearing - they have been spinning back there for 23 plus years! Could also be a bad outer or inner CV joint. How's the boot's? Leaking? Even with motor off and trans in neutral the differential is still spinning - could be a bad carrier/axle bearing. Start with the axle nut and work your way inward.

I think it is the CV joint. I noticed a clunking noise this afternoon as I was taking off. And it happen again a few minutes ago so I am thinking that maybe the problem

I have replaced the rear axle, rotated and balanced tires (Again!), changed out motor mounts, transmission mounts and the vibration is still there. I can turn off the motor, knock it out of gear, let off the clutch and the car still vibrates so I don't think it is transmission or clutch related

Any suggestions before I roll it into the back corner of my shop and forget about it

Robby

[This message has been edited by RobbyH (edited 06-30-2011).]

If you are balancing the tires off the car you may have an uneven hub or something that is preventing the wheel from sitting right, either on the wheel or hub. It sounds like a bent wheel, but that would be picked up in the balancing.

quote Originally posted by katatak: Check your axle nuts! If one is loose, it will cause a vibration - eventually will destroy the wheel bearing. The axle stub/nut holds the rear wheel bearing together. Even if the axle nut is tight, it coud be a bad bearing - they have been spinning back there for 23 plus years! Could also be a bad outer or inner CV joint. How's the boot's? Leaking? Even with motor off and trans in neutral the differential is still spinning - could be a bad carrier/axle bearing. Start with the axle nut and work your way inward.

I vote for untorqued axle nut. I took my axle nut off and don't think I torqued it down correctly. When driving I get a swaying and lots of vibration motion on the side I had removed.

[This message has been edited by sricka01 (edited 07-01-2011).]

If the frequency of the vibration is independent of engine speed, then the culprit is either the differential, the axles, the wheel bearings, or the wheels. Given what you've done so far, I rather suspect it's your wheels. You can swap the wheels front to rear and see if the problem shifts to the front of the car... you'll feel it in the steering wheel. If the vibration doesn't change ends, then perhaps it's the way you mount your wheels on the car.

If you have aftermarket wheels where the hub hole in the center of the wheel is larger than the snout on the hub flange, then you need a concentric ring adapter to get the wheel mounted concentrically on the hub. Without the ring adapters, a mis-match in the manufacturing tolerances in the wheel holes and/or the wheel studs will allow the wheel to be pulled towards the first nut you tighten, if you're using standard acorn nuts. This offsets the wheel a tad and gives you vibrations even though the wheel and tire assembly by themselves are balanced.

Things to check:
Axle(s) nut torque to 200ft lbs is the required torque
Rear bearing hubs
Bent wheel
Defective tire
wheel alignment
CV axles
rear tie rods


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[This message has been edited by Dennis LaGrua (edited 07-01-2011).]

My guess is in th cv axle(s) simply because sometimes when doing transmission work they can be played with or if replaced could be not balanced correctly. Cv axles are about the same as a driveshaft and need yo be balanced correctly. I've bought a drive shaft for my old car and they lift their cars using a giant forklift which left a very small ding in it causing higher speed vibrations. If you replaced them it could be a bad axle that was miss handled or not balanced correctly. Also check over the struts in the rear and make sure the mounts are still good and not worn out. Hope you figure it out no fiero likes to be locked up in a corner of a shop unless being worked on