Vibration Under Heavy Load

[GoatRope]

Getting a pretty decent vibration under heavy load, say in excess of 5 psi of boost, worse in lower gears, though will still have some shimmy in 3rd if you load it up. So far I have replaced/upgraded the following:

Front half shafts and ball joints
Heavy Duty prop shaft
Built 4l60E with FTI converter
Driveshaft u-joints
New plugs/wires in wasted spark configuration, no misfires

It almost seems like axle wrap causing pinion angle to get out of whack and create driveline vibrations, so I put some spring clamps on but couldn't get them over the overload springs to really tighten it down. Pinion deflection with a jack was only 3.5 degrees. Spring clamps acted like they might have suppressed some of the issue but it is still present.

Any insight/suggestions are appreciated!

 

[gmemony2]

Is the truck lowered at all? I know you didn't list it in the mods, but wanted to check.

 

[GoatRope]

No, but it looks like I’m blowing though the converter pretty hard so I’m going to swap it and see if that’s the problem

 

[GoatRope]

New converter, not blowing through this one, but still vibrating under hard acceleration. Very annoying.

 

[Slyclone]

I'd look into wheels and wheel balance. If all check out investigate the transmission mount/transfer case connection. Could be something off or loose. I would continue to go through the process of elimination.

 

[GoatRope]

Wheels have been rebalanced and I’m picking up a new trans mount today. Will report back

 

[Slyclone]

Are you using a poly type mount? Factory mount?

Years back I discovered a transfer case connection bolt head snapped off. The hardware is actually still inside the adapter/flange I was never able to get it out of the part. Welding shit on it never worked. Had to get a whole new adapter. Old one is still floating around.

Those poly type mounts have a strange extra plate that can cause some confusion.

How are the shocks doing? New/old some type of hybrid replacement system? shock towers look good and the hardware all tight.

A lot of the times a engine mount can cook causing some play in the engine. Investigate the mounts well look them over front and back look for high heat indicator. Sagging mount will eventually sever the radiator hose off look for fan blade location close to upper radiator hose.

Not sure if I have any other ideas something may just pop out and you will figure it out

 

[GoatRope]

Engine mounts were replaced with factory style mounts, but trans mount is still old, so that was my next guess. Shocks have been replaced at some point with Bilstien units, they might be a little weak.

I’m going to check drivetrain angles when I’m under it to do the trans mount today, it almost feels like pinion angle issues due to axle wrap.

Should probably add that it has a 25g turbo, 3 inch cat less exhaust, vortec heads, some other go fast mods.

 

[Sy2622]

Check to see how true your wheels are.
These trucks have a tendency to make them out of round. My rear wheels are just out of round enough to have a harmonic at 70mph.