bad vibration problems

the rear end then? stay in the power band longer reminds me of the 2 stroke dirt bikes i ride.gotta keep them there for that rush-

 

I am leaning towards a bad injector that is only missing when under load, and at a certain RPM range. I have seen this before. You basically have to go on a road test with a mechanic while he has a laptop connected. Pulling a good heavy trailer, get to the RPM range that is causing it. At this time the mechanic will cut out cylinders one-by-one and hopefully find the weak or missing cylinder.

 

Anything driveline related will usually only play up at a certain speed, not rpm. Dump your airbags as you are driving along, this changes the driveline angles, if the vibration changes it is a bad u joint or something driveline related.

I'm been chasing a bad vibration in a 6 month old truck since the day I picked it up, Kenworth, cummins and eaton have all looked at it many times and have no idea. Only does it under full throttle. It has had 2 faulty clutches, the first was heavy north and south on the buttons and would not beak torque, the second had an unbalanced pressure plate. It is in now getting its 3rd new clutch, but what makes it more frustrating is the truck is a twin steer, so it is a 3 day job to change the clutch. while they are doing the cluch they are also having the drive shaft balanced, the pto shaft balanced, flywheel machined and balanced, multiple injector cut out test have been done, they are also looking at the rear engine power take off and seeing if the backlash and bearings are alright, they are checking the end float aswell.
If none of this will fix it they think it has an unbalanced crank,
So don't be disappointed if they can't find it straight away as it can take along time to find a vibration.

 

I have a friend who spent a lot of money rebuilding truck components in an effort to get rid of a vibration. In the end the problem turned out to be the barometric pressure sensor. I would check or change the cam speed sensor and such .

 

To test a dampner, remove it, bounce it on the tire from about 4 inches if it rattles it is junk, if it is bent or visably bulged it is junk, other than that it is good.
Jack up the rear wheels and check the wheel bearings are tight and the weights are on the brake drums, the tire rims are not bent, the tires are not out of round more that .050.
With one wheel up and the transmision in gear you crawl under the truck and look and feel the "U" joint cups, get someone to rock the tire back and forth. Check the center bearing if the rubber shows signs the bearing has turned in it, replace it, check the motor mounts, buy a new differencial magnetic drain plug and do a drain plug swap looking for chunks. On the diffs and transmision. Get the right specifications for the ride height and adjust it properly. Inspect every bolt, every "U" bolt, retorque them, every crossmember for cracks, look at the torque arms and the bushings for slack and cracks at the mounts.
Just a thought!

 

in a cat engine if that is what you got. a bad injector can blow smoke, but also not. it could be at a certain rpm load an injector isn't seating properly in the plunger body or there is dirt in the actuator assembly causing the vibration(hence a weak injector) that is throwing the engine off balance when the cylinders are firing. the cut out test isn't to reliable unless someone has had experience doing it. been there done that! I have not found a cat shop that will do cutout test outside of cats requirements. not saying this is the problem. there are so many things that can. but a unbalanced fuel system can cause this with no visible smoke out the stacks.

 

put a inline sight glass in the line to see if there is bubbles.

 

You could also put some miles on the oil and take a sample. Cat an do this for you and it will only cost about $20. You could catch signs of a bad injector that may not be resulting in smoke.