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Old 04-26-2010, 09:23 PM   #1 (permalink)
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Default What could be causing this vibration?

So in the past couple of weeks I have noticed a vibration while accelerating. It feels like it's coming from the rear of the car. Also, it seems to be an intermittent problem because it only does it 75% of the time or so. I had the wheels balanced because I thought they could be it but it did not fix the problem. I thought the U-bolts on the rear U-joint might be loose so I took them off and then put them back on nice and tight. I thought I might have found the problem since one of the nuts was loose. Still didn't fix the problem. I'm kinda stumped at this point. Any ideas?

The rear-end (8") was just rebuilt a couple years ago, I believe the only original things are the axles, everything else is new. I just had the driveshaft balanced and new U-joints put in last summer.
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Old 04-26-2010, 09:47 PM   #2 (permalink)
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Did you check for slack in the U joints.
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Old 04-26-2010, 10:27 PM   #3 (permalink)
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What kind of transmission? I had a similar problem when I installed my toploader, turns out the input shaft was too short and was not seated properly in the pilot bearing. I could still drive the car and operate the clutch, however, occassionally vibration/axle hop would occur under load. I had the input shaft replaced with the correct size and all is well...no hop, no vibration, just smooth accelleration :-)
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Old 04-26-2010, 10:59 PM   #4 (permalink)
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That really sounds like a drive line issue.
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Old 04-26-2010, 11:34 PM   #5 (permalink)
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Verify slip yoke to transmission rear bushing clearance, along with proper insertion length. If out of spec, even if not 'loose', vibration can result under power, though it might be imperceptible at cruise or decelerating.
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Old 04-27-2010, 10:17 AM   #6 (permalink)
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Does the vibration occur at the same speed while accelerating and decelerating?
In other words if you go past the speed where it is vibrating and let off the gas does it vibrate as it passes that speed on the way down? If so do the same and then put it into neutral and let it coast down.

If the vibration varies with the above tests then you almost certainly have a bad U-joint or the tranny yoke is severely damaged. I'll bet when the u-joint nut was loose it beat up the needle bearings and the u-joint is trashed.
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Old 04-27-2010, 04:35 PM   #7 (permalink)
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Transmission is a C4 and all the parts are original to the tranny (besides the seals and other stuff that is changed during overhaul). I'll get under in a few and check yoke to tranny bushing clearance, but where do I find what the proper insertion length is?

68RCodeConv, it only does it on acceleration, never cruising or deceleration. Almost exactly as camachinist describes it.

There weird part is, it never did this except for the past couple of weeks which leads me to believe it's not what camachinist describes only because I would assume it would have done that immediately after having the driveshaft balanced and what not a year ago. Maybe I'm wrong?

Does anyone know the insertion length? I guess a better question would be, how much of the yoke should be sticking out of the tranny?
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Old 04-27-2010, 04:50 PM   #8 (permalink)
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Given that it just started and that you found a loose not on the rear u-joint I would concentrate there.

I had a severe vibration right around 72 MPH that I couldn't find to save my life. Turned out to be the wrong rear u-joint for the yoke in the new rear carrier I bought. Turns out that the universal wasn't centered between the lugs. As a temporary fix I used washers as shims but will replace the u-joint this spring. My vibration was at speed, but I would have sworn it was from the front end!

You never know...
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Old 04-27-2010, 05:06 PM   #9 (permalink)
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So I check and the tranny yoke seems to be fine where it enters the bushing. Also, there is roughly 1 3/4" of the yoke sticking out of the tranny.
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Old 04-27-2010, 05:14 PM   #10 (permalink)
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I usually work for ~1 inch of stickout from completely bottom (inside or outside) but that's in applications with minimal suspension travel, like racing. The amount in the picture seems high to me, but perhaps more indicative of OEM. Radial yoke to bushing clearance should be about .0025". I found, once radial clearance approached .005", vibration began under load and/or high rpm, generally over 6500rpm. Tighten the clearance up, vibration goes away.
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Old 04-27-2010, 05:15 PM   #11 (permalink)
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Don't forget to check other common culprits while you're snooping around under the car, specifically shagged-out tranny and/or motor mounts...
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Old 04-27-2010, 07:18 PM   #12 (permalink)
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Worn-out parts are unlikely as I just finished restorting this car so everything is new. I would guess more likely that something is broken or loose. I'm still not convinced that it doesn't have something to do with the rear u-joint but I'm not really sure what else to do besides replace it. From the exterior it looks just fine.
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