1999 peterbilt 379 ext hood bounce wobble

I agree with what special k says. After everything you have done, don't give up now. I'm afraid I can't come up with much more to help you, except to say that I've seen u joints that appeared to be fine, and actually weren't. You've got 5 in there right? I'd be looking at replacing any of them that haven't been touched in the last, say, 100,000 miles just to be safe. Not a great expense compared to all else you've done. IMO well worth trying. Other than that maybe you'll get some ideas from some of the mechanics who';ve been around for a long time, such as "wore out". Keep at it and good luck!

 

Warren, I feel your pain. I have had the same problem for over 3 years, NO one can diagnose it. I can't even remember exactly when it started. I replaced everything as you have. the money was not really a waste cause replacing things like the shocks at 12 years old, was due anyway, new tires when I needed them, e.t.c. sometimes I think it is the wind resistance, tire, rim , just do not know. the bouncing shakes so much other things loose & thinks break from all the bouncing. I have a few shops that are good to doing quality work, but I know of NO one who can properly pin point the bounce I have.

on my truck if I can hit 72 m.p.h. the bounce just about disappears. I know certain loads make it better or worse. I would gladly pay a thousand dollars to find & fix the problem. when I fist had my kelly K.D.A. lugs, I thought that was the reason. thought all kinds of replacements would solve it. not one even reduced the bounce. I may try to have my wife drive next to me & video the truck moving & then try to determine what is going on. let me know if you ever find the solution.

 

If the vibration is in the floor and seat at a steady drone it's most likely driveline. I have seen u-joints lock up but still not have slack from spitting the needles out. If you feel it in the steering I would check all bearings as Marmonman said. Also flip your drums around backwards when the wheels are off to make double sure they are seating on the hub. I have seen some lately that wouldn't and caused the wheels not to pull up even, with a big enough gun that will show itself lol. I had better luck on my old 359 with equal for tire balancing, not sure which way yours were done. Also there is an adjustment on the steering box that can cause it to feel loose headed, sometimes it will cause a shimmy. If you turn it yourself always mark it so you can go back, usually on older trucks in is the answer no more than half turns at a time. You can get it to positive and make it hard to hold in the road as well. Vibrations and the like can be the worst things to chase for sure.

 

Go to east Texas truck alignment in Marshall Texas. They will fix it for you. If your alignment was done on one of the computers I wouldn't trust it. These guys do it the right way. They will also true and balance the steer tires on the truck. This balances the the tire, rim and drum. They can also true the rear tires. You wouldn't believe how out of round a brand new tire can be. If you have a week stomach you may not want to watch them true a new tire. But it will last much longer and ride much smoother. Afterwards they will test drive to make sure it is right. After taking mine there it is the smoothest riding truck I have ever been in. They are just a few miles south of I 20 on highway 59. They really impressed me.
i would also take the drive shaft out and take it to a shop to be balanced. And inspect the u joints with the shaft out. Might as well replace the carrier bearing while the driveshaft is out.

 

what about finding a way to place your cell phone on video and mounting it in different areas under the truck and videoing as you drive. Or if you want to spend a little on of those hero cams.

 

That's an idea. thanks.

 

Another thing that gave me some vibration were the bushings in the springs themselves. Front and rear. And when you changed the faith wheel did it get new bushings also. I also think a lot of it is in your tires due to it not being as bad after they get warmed up. I have an old 379 like yours with about as many miles.

 

Its hard to say, I do know that if the spider gears and pin are worn it will cause a vibration under acceleration at times. Turn that yolk real gentle back and forth, see if you can feel the ring and pinion lash. If you can feel the back lash in the ring and pinion then the rest of your slack is most likely in your spider set. Course it needs to come out any way so seeing it on the table will tell the tale.

 

I have tried several height adjustments on my leveling valve, by bending the rod, all it did was move the bounce to different speeds.