Rear end vibration

[K20Detroit]

The truck is an '84 K20 diesel, 700r4.
Smooth until 55mph, then vibrates in pulses.
I have been able to recreate this on a lift in 2wd.
Still does it with tires off, so not that simple.

I have;
Rebuilt rear axle. New shafts, everything but the housing and ring+pinion are new
New Spicer u joints
Balanced driveshaft
No play in the axle, t-case.

Any ideas?

Edit; it is not gear dependant. 3rd and 4th, as well as 2wd or 4wd, do it the same.

[Irish_Alley]

one guy found out it was prob worn splines on his driveshaft
http://forum.73-87chevytrucks.com/smforum/index.php?topic=12389.msg282362#msg282362

know another found out his trans was the culprit
http://forum.73-87chevytrucks.com/smforum/index.php?topic=33409.0

[K20Detroit]

Forgot to say, it does it coasting in neutral at those speeds.
The splines felt good. No play, slid in firmly.
The trans, i know has issues. I bought the truck right after the PO had a line blow. It would not shift when i bought it.
It does alright, but 4th feels soft.

[bd]

One-piece driveline?

[VileZambonie]

Is the crossmember and mount in good shape? 84 crossmembers were known for cracking. Is the crossmember in direct contact with the frame or are the spacers between the frame and crossmember? Is it lifted or stock height? Did you check the pinion angle? Did the vibrations stay the same after you rebuilt the rear end and replaced the u-Joints?

[K20Detroit]

One piece driveshaft, stock height.
Pinion angle appears to be correct.
I have had this truck 4 years now. It was 3 years ago that i had the rear rebuilt and duralast u joints installed.
For a while, it was smooth as silk. I don't know when this vibration started. I had a goosed front tire, a wheel bearing on the front go, and other various front end issues that caused other vibrations in the mean time.

This week, i installed the spicer joints as a precaution, after hearing how bad the duralast joints are. The u joints made no change.

I have not checked the crossmember. Where is it known to crack?

[VileZambonie]

Under the mounts. A block of wood and jack will reveal if its sagging which in turn will throw off your driveshaft angle.

If you rebuilt everything and it was smooth as silk but now has the same vibration 4 years later, that doesn't mean you can eliminate the rear end from being the issue again. Have you removed the cover and inspected? Look for metal in the oil, pitting in the bearings, check gear tooth contact pattern.

[K20Detroit]

Under the mounts. A block of wood and jack will reveal if its sagging which in turn will throw off your driveshaft angle.

If you rebuilt everything and it was smooth as silk but now has the same vibration 4 years later, that doesn't mean you can eliminate the rear end from being the issue again. Have you removed the cover and inspected? Look for metal in the oil, pitting in the bearings, check gear tooth contact pattern.

The axle was rebuilt due to a bad pinion bearing, it is not the same kind of vibration.
The rebuild was done by East Coast Gear Supply, and a follow up inspection was done a year later.
Supposedly everything is fine there. The fluid still looks new, i dont drive the truck much anymore.

A broken mount/warn mount sounds highly possible. It has 249k miles and has never had the trans or engine removed.

I will start with the mount before pulling the cover.

[K20Detroit]

Got under the truck, here is what I found.
Anyone know the threading? The threads look to be in good shape.
If not, the bolt goes all the way through and I can nut/bolt it together.

Other than that, the mounting feels solid. No cracks, no movement when I jack up trans, but I imagine this is a big contributor to my vibration and harsh shifts.

[bd]

The bolt should be 1/2" x 13 TPI x 3-3/4" Grade 5.  With all of that oil, those upper saddle mounts maybe getting soft.  Locate and repair the oil leak, replace both upper saddle mounts and the missing lower isolator and plate.

[VileZambonie]

https://www.summitracing.com/parts/ens-3-1106g/overview/

[K20Detroit]

The bolt should be 1/2" x 13 TPI x 3-3/4" Grade 5.  With all of that oil, those upper saddle mounts maybe getting soft.  Locate and repair the oil leak, replace both upper saddle mounts and the missing lower isolator and plate.

The oil leak is actually PS Fluid, on top of old oil from the past 34 years. It's on my list of things to fix.

Any idea on where to get the lower isolator plate?
I can get the isolator for about $3, but don't see a way to get the plate itself. It seems like it would be very easy to make, though.
In fact, I likely already have something in my scrap pile that will work.

I see the energy kit. I'm not sure if it is worth the extra $$ on this truck over OEM, it's not like I am putting out that much power.

[bd]

Urethane is impervious to petroleum oils.  IMS, the plate is integral with the factory style rubber isolator - it's one piece.  At the very least clean and check the upper mounts for swelling and firmness.  If they are the original mounts, they are probably toasted.

[K20Detroit]

Urethane is impervious to petroleum oils.  IMS, the plate is integral with the factory style rubber isolator - it's one piece.  At the very least clean and check the upper mounts for swelling and firmness.  If they are the original mounts, they are probably toasted.

If this mount fixes the vibration, i will probably fork over the money for poly mounts.
I checked, the uppers seem firm. They are swollen for sure, though.
They don't 'feel' like 250k mile mounts. I have seen some much worse on much newer cars.
In fact, that may be what happened. New mounts installed, improperly torqued, bolt rattled out.
It's hard to tell in the pics i see online. I will pick up the mount and see if it has the steel with it.
I will also try the 1/2" " 13 bolt. Some sources online said m10 1.5, that is a tad small and i didnt think it would be metric.

[K20Detroit]

New mounts in, feels a little better.
Question;
There is a 1" block under the rear leafs. I have been told this is a factory thing, but that doesnt seem right.
They are not angled at all.
This truck had a 1" block on the front. They were spit out on the highway years ago.
I never took the rear blocks off, as i was told they are original.
They are alloy blocks, don't sound period correct to me.
Any insight?

Speaking of that, i dont remember any vibration when the front blocks were in.
The blocks were in for about a year after the rear end rebuild.