Drive Shaft: Driveline Noise on Heavy Acceleration

[CaffeineRacer]

Well I done it again. I accelerated from a stop a little hard and I heard what I though we're just the tires slipping a little bit on some gravel. Now however any time I accelerate a little hard it makes that same noise which is now much more like a grinding noise. Like gears slipping. If I drive around with caution and accelerate lightly no noise. Car still shifts fine which leads me to think no issues in the transmission. Im thinking maybe the splines on the drive shaft the go into the differential pinion gear broke.

Any suggestions, thoughts or good ways to go about diagnosing?

[studegary]

Does your car have Twin Traction (limited slip differential)? It could be the clutch discs in the TT.
My other thought is universal joint noise due to wear and/or lack of lubrication.
If you tell us the basics, like what the car is and how it is equipped, we may be able to help you more.

[CaffeineRacer]

Oh yeah that might help. Haha. It's an 82 Avanti with positraction, 2.87 rear gears, 2004r transmission, 350 sbc, and about 58k miles on the odometer.

[studegary]

Oh yeah that might help. Haha. It's an 82 Avanti with positraction, 2.87 rear gears, 2004r transmission, 350 sbc, and about 58k miles on the odometer.

With that, I will stick with my first "guess" of limited slip noise. Have you changed the differetial lubricant? Did you add the special limited slip additive (a small bottle - GM has one for posi)?
I see that your Avanti is modified since it originally came with a 305 V8. Those 2.87 gears seem pretty "tall" with an overdrive. Did you change the differential from the limited slip that it came with to a GM Positraction unit?

[CaffeineRacer]

No it's the original rear end. Sorry I always associated positraction and limited slip as being the same thing. The diff still has the factory tags on it with the gear ratio and the tag saying to use only limited slip gear lube. I changed the gear lube last year and made sure to use the correct stuff for limited slip rear ends.

[rockne10]

Could also be as simple as cracked motor mounts. Open the hood and accelerate with the brake on hard. See how much the engine moves.

[CaffeineRacer]

Question, if it were an issue with the LSD wouldn't the noise also be present when turning.

[rockne10]

...an issue with the LSD ....

Can't help but chuckle.

[bezhawk]

Can't help but chuckle.

But the flying monkeys would drown out the noise one would think?

[PackardV8]

One more vote for U-joints.

Counter-intuitively, the tall 2.87 gears put more strain on the U-joints than would a lower gear.

jack vines

[CaffeineRacer]

So the real question is do you need LSD to work on an LSD? Lol

I will check the U joints. I assume to check u-joints you just need to pry and pull on the driveshaft to see if there is any play in them?

One suggestion I had from a friend was that it was perhaps the clutch packs in the differential. Any ideas for how to check that?

[CaffeineRacer]

Jack, that makes total sense to me. It's like trying to pedal a bike from stop in a high gear. You transfer a lot of torque to the gears and chain just to get them moving.

On a side note, if I end up having to rebuild the diff what would be a good ratio to go with. I was thinking of something around 3.11 which would be a little peppier for city driving but still keep the RPMs down on the highway with the OD tranny.

[PackardV8]

Jack, that makes total sense to me. It's like trying to pedal a bike from stop in a high gear. You transfer a lot of torque to the gears and chain just to get them moving.

On a side note, if I end up having to rebuild the diff what would be a good ratio to go with. I was thinking of something 3.11 which would be a little peppier for city driving but still keep the RPMs down on the highway with the OD tranny.

Going from 2.87 to 3.11 is just one step down; noticeable but maybe not really worth the cost. Usually, it takes two or three steps down, to 3.31 or 3.54 to be enough of a difference to be worth the cost and effort of new gears.

Anyway, it's unlikely that few miles has damaged your R&P.

jack vines

[njonkman]

My guess is that you have a U joint full of rust and groves from the bearings. I have seen that hundreds of times in my 55+ years as a mechanic. They will give you that sound. If you get under there you can probably see the rust coming out of the joint. Otherwise just pull the shaft and feel the joints. The bad one could feel like it is ceased too. The bearings may just be crunching in there.
Nick

[CaffeineRacer]

So I replaced the u-joints (spicer pn 5-153x). No such luck. Still have the noise during heavyish acceleration. Ithink was sort of a long shot since I did find any play in the old ones.

Here are my new findings.
1). I felt a vibration/clunking under my feet one time when the noise was occurring.
2). There is about 1/32 or maybe 1/16 play in the output shaft on the transmission with the driveshaft in place. Is this normal or should it be rock solid?
2). With one rear tire in the air and the other on the ground and the car in neutral. If I turn the tire in the air by hand there is a little resistance at first but then the tire will rotate indecent of the one still on the ground. Is that normal or are the clutch packs in the diff shot.
3). There is plenty of fluid in the diff.
4). Sticking a magnet on a stick into the diff yielded no metal fragments or really an metal particles at all.

Next things I'm going to check are the transmission mounts.

Any other ideas?