I just can not figure out my vibration problem!!!!!
97, LT1, M6 49,000 miles.
Vibrates over 70mph worse as speed increases, stays the same with the clutch in.

I have swapped dshafts for an LS1, had it checked for balance and straightness. It was in spec.
Dshaft runout installed in car is .012 at both ends of the shaft, spec is .040
I also checked for output shaft play and slip yoke slop. It is as tight as any other car I have.
I installed a complete different rear, still vibrates.
Tried different wheels with new tires, still vibrates.
I have run the car on jackstands without tires, rotors, axle shafts, carrier and it still vibrates.
I put in new pinion bearings and races, rotating torque of 20inlbs and tried two different pinion yokes, still vibrates.
Took out the poly trans mount, it still vibrates but it is masked by the softer mount.

I've never run anything but street tires on this car... Is it possible the output shaft is bent? Is there some way to measure for that?

What the HELL else could it be?!?!?!

 

There is the possibility that it could be the engine vibrating, due to a bad balancer/pulley or some other issue. Does it vibrate in all gears at that speed, or just sixth? You could try it in third-sixth and see if anything changes. It's worth a shot at least.

 

Its definitely speed related, any gear over 70-75mph.
I can push the clutch in at 90 and the vibration stays the same. So I'm pretty sure it isn't engine related.
I just can't think of anything else to try?

 

Did you run it on jackstands with the driveshaft out? If not try it and see if the vibes go away. Your're on the right track, just need to isolate more.

 

I ran it with the driveshaft out, with a slipyoke in the tranny, and didn't feel any major vibration.

This is where I am confused. I don't know what else there is to do.

I ran the car on jack stands with no carrier, so all thats left is the pinion, pinion yoke, driveshaft and output shaft. Bad vibration.

To correct it I have tried installing my stock steel shaft with new Ujoints. Still Vibrated.
LS1 shaft with new Ujoints. Still vibrated
New pinion bearings and different pinion yokes. Still vibrated.
Measured the driveshaft runout, on the tube, .012 on both ends, the high spot was in the same position at both ends.
Dropped my exhaust to make sure it wasnt touching something. Still vibrated.

I haven't checked my pinion angle because it is stock height, stock torque arm.

I can't think of anything else to check. I was pretty sure I had it isolated to the pinion, pinion yoke, drive shaft, Ujoints or output shaft. But I think I have tested most of that pretty well.

If the driveshaft was twisted so the Ujoint ears weren't aligned, would a balance machine see it?

 

There are a couple of things I would check still: The pinion angle and slip yoke play. I know you said the slip yoke felt tight, but how did you gauge it as tight? Did you try twisting it as well as pushing is from side to side? Does the slip yoke show signs of excessive wear? It could be the bearing inside trans that the S/Y rides on needs replacemt too.

The pinion angle should theoretically be the same, but with the car being ten years old, the mounts may sage some or replacement mounts can be at different heights than OEMs. I would check pinion angle to rule it out.

 

Good point..... I should really check the pinion angle. I don't have the stuff to do it. But I can get it.
How common is a bent torque arm on a stock car? I can't imagine I hurt it, but is it possible?

Both slipyokes LOOK ok, but that is just a visual inspection. I could only get a little movement up and down with my LT1 slipyoke. The LS1 slipyoke had more up and down play but neither had detectable twist play. I'm currently using my LT1 yoke on the LS1 shaft. (vibration is the same with either yoke)
I was going to try to get a runout measurement right behind the dust seal on the slipyoke, but I'm not sure if I'd get true readings because of slipyoke play.

Thanks guys, if you have any ideas, please let me know. I got the 4.10's in the new rear and it sucks to have to shut it off at 90mph cause of the vibration.
My original 3.42 rear vibrated as well, but at about 10mph faster (taller gears)

 

I have a similar problem with my Camaro. At 70-75 mph everything is fine, but once I reach 80 -85 my car vibrates like crazy. The driveshaft on my car is an OEM replacement with only 3k miles on it, so I don't think that is the issue. However, I have ES Mounts and an aftermarket torque arm, so maybe I should get off my a-s and check my pinion angle

 

That info pretty much eliminates engine and rear as causes, so I'd say your in the right area.

Of all the things you changed behind the trans what has stayed the same?
You mentioned it being possible to bend the factory T/A...being stamped steel it is very possible it could bend without breaking and throw off pinion angle.

How much slip clearence is there in the yoke at ride height? If there is too much or too little slip... say from moving the rear with adjustable LCA's it can cause the driveshaft to vibrate.

 

The things that have stayed the same are; driveshafts, torque arm, tranny output shaft/bushing. I'm pretty sure everything has else has been changed. (Stock rear suspension, but running it on stands should have eliminated that I think)

With the LT1 slip yoke installed at ride height there is 1.25 inches until bottomed out in the tranny. The LS1 yoke is slightly shorter, but the measurements from ujoint to ujoint is the same on both the LT1 and LS1 shafts. So the LS1 shaft with LT1 slip yoke should be "factory" length. It does fit nice and snug with the LT1 yoke. (rattled the yokes on my truck, wifes Mustang and Jeep for comparison, it feels the same)

One thing that is constant is I rebuilt both rears. Could my assembly technique be a problem? (I don't know how? tightened pinion yoke with an impact?)

I bought a angle finder tonight. Hopfully I can check the pinion angle with in the next few days.... I am gong to use this guide.
http://www.streetissuecustoms.com/pinionangle.html
If I'm reading it correctly, I can hang the angle finder off the output shaft and then the pinion (with rear suspension loaded) and compair readings to find the angle.

At times I want to push this car off a cliff!!! It just bugs me I can't figure it out!

 

Did the vibration start after rebuilding the rear the first time?

 

I think so... I'm not sure, it was in 2005. I wasn't driving it that much (or very fast) at the time. It was definitely more pronounced after I dicked around with it.

I tried to put in used 4.10's in my original housing and failed. I couldn't get them to stop whining..... They may have been junk, ebay special.

This second rear is totally complete (no parts reused from the original rear) The 4.10 gears came out of a 95 S10 and are dead silent.

I'm not sure how I could have screwed it during installation?
I'm hopefully going to get a pinion angle measurement tomorrow.

 

I checked the pinion angle. It is -1. I measured off of the back of the tranny output shaft and got 0. Then I measured off the end of the pinion and the underside of the torque arm mounting point on the rear. Both read 1 in the downward direction.
The driveshaft showed 1 in the downward direction (-1)

The torque arm "looks" straight and damage free as does the tranny torque arm mount bushing.

What else am I forgeting to check?

 

Well the pinion angle as measured is good and probably at factory spec.

I just noticed that the slip yoke travel is 1.25in. I think it is too much, could be wrong but it should be around 3/4in.
1in max for high horsepower applications that launch hard.

 

I agree. But this is factory stuff? I'm curious if all stock LT1 driveshafts have that much travel?