Pulsing vibration

[terrywens]

Need some opinions.
I have an 87 4x4 short bed, 4" lift, 295/70/17 tires, new long block, rebuilt tranny, rebuilt transfer case, new u-joints, and a new front end, tires are balanced.  shocks are not leaking but pretty rusty and old looking and i've had the covers off the differential and there are a few chipped teeth on the ring gear. I'm slowly working my way thru the truck.

the problem is at approx. 55mph and up there is a vibration somewhere in the driveline. it pulses in and out approx. every 15 to 20 seconds but randomly it will smooth out for several miles the longer the road trip is. i cant tell where it's coming from. It rides like a brick so i've got about 200 lbs in the back to help calm it down a bit. (that will be another fix coming).

thoughts?

thanks a lot,
Terry

[zieg85]

Welcome from NW Indiana...  Have the same thing going on with my 85C20.  I can tell you I've gone through everything looking for the problem.  I think I have found the issue being one or more u-joints being seized or rough spots in them.   There is no play but I bet they are rusty and binding when going up and down and when I load the truck down the u-joint are straight and the vibration is gone for the most part.  My truck is not being driven and is in the middle of a face lift and before final paint I plan to resurface the flywheel and treat it to a clutch and pressure plate if needed and a throw out bearing for sure.  At that time new u-joints if I find that is the issue.  Good luck with yours.

[DnStClr]

Nice truck Terry, -Another possibility for the vibration is a brake shoe hanging up a little bit. You'd have to raise the wheel and spin it, checking for it.
Were all the u-joints replaced?
 With the truck lifted, you might be concerned that the drive line angle has been changed, which could also cause vibration and u-joint failure, altho 4" isn't really too extreme is it? But still, anything more than 3-4 degrees can cause problems.  A decent drive line shop can check it.   

[terrywens]

Thank you very much for the compliments, It is fun to drive.

Thanks for the ideas, it's interesting that when I bought the truck a couple of years ago it went down the road as smooth as could be. Now that i'm doing all this fixin up is when the problems start,,,,

The U-joints are brand new and we just did a brake job and replaced everything all the way around. the vibration was exactly the same before and after so I don't think that's the problem.

I've thought about the drive shaft being out of whack but nothings changed since the U-joints were put in, it didn't vibrate at all after they were in. Could it be the shocks? It's honestly hard to tell if the shocks are bad with the truck being lifted the way it is. It's got  a beastly leaf spring on each axle holding the truck up so there's not much flex at all. I've had light loads on it but with the bed full or empty it feels the same.
I also cancelled the idea that it was the wheels and tires, we balanced and rotated them last week but there is no difference in the vibration even after rotating the tires.
I'm somewhat thinking that it might be coming from the differential after being told it was pretty sloppy when the cover came off when the axle seals were changed.

definitely perplexed,,,

[86454]

I am not familiar with 4x4s but my 85 2wd had some vibration issues and changing the transmission mount on the cross member solved the higher speed vibrations. The rubber was cracked and worn and I didn't realize how worn it was until I took it out to replace it.

[DnStClr]

Wheel brgs also comes to mind-and yes- shocks too. I had steering vibrations with bad front shocks. 

[bd]

...The U-joints are brand new...

Any possibility a u-joint bearing cap isn't fully seated in the yoke?  Are the yokes tight?

[terrywens]

Transmission and motor mounts are new so I crossed them off my list, wheel bearings were checked last week when we put rear seals in and they were in good shape. I'll recheck the U-joints, good idea.

I think what i'm going to do is start with the shocks and go from there. Because of the way it's lifted the springs aren't letting the shocks really move at all. And even on good smooth asphalt when I ride over a seam or a crack the truck wants to shudder a bit more than i think it should. I might even have some bad bushings,,,, So even if the shocks are not leaking only a very small portion of the piston in the shock gets any use, and i've been told that can cause the shock not to work as it should. We'll see if that's true.

I'll keep you posted, thanks for the ideas!