After receiving my truck earlier this week, I went out to drive it around normal roads and everything was ok. I took it on the highway last night and go it up to around 55-60 mph and noticed when I would let off the gas and not brake, it felt the like front end of the car was sort of bouncing up and down and vibrating. At slower speeds when braking there is no sort of vibration such as a warped rotor. Any idea what could be causing this and what to look at first?

 

Check the U-joints.

If you can put it on a lift with the wheels off of the ground, and get it to duplicate the problem, someone standing underneath might be able to spot the cause.

 

I did read some on that last night, but was related to new model trucks. Is there anything in particular I should be looking for on the U-joint?

 

Grab the driveshaft and try to wiggle it up and down or side to side. See if the U joint has any slop in it. It should be pretty firm.

 

Year? RWD or 4WD? If the latter, part time or full time 4WD? Check to be sure lug nuts are tight. When you let off gas try kicking it in neutral and see if it still does it. I don't like the sound of it. I would not drive it other than for testing until you solve the problem.

 

It has a little bit of play but much unless really pulling on it. They lookin like they havent been touched or replaced in years. It wouldn't hurt to update/upgrade them. Do you have any recommendations for sourcing a new one?

 

Its a 76, 4wd and part time. When you say kick it into neutral are you referring with the transmission, or the transfer case?

 

Transmission into neutral. If its doing it with transfer case in 2WD and hubs unlocked, theoretically that should eliminate front driveshaft, front differential and front axle u-joints.
You could also try the following
First make sure the hubs are physically unlocked. Not just turned to unlock but jack up front axle, spin wheels and make sure the front axle shaft does not spin with it.
Transfer case in 4WD and hubs unlocked. Then transfer case in 4WD and hubs locked just to see if you get any different response which might help direct to the root cause.
Letting off the gas at higher speed and no braking shifts the strain of driveline in the opposite direction starting from the rear wheels forward through the differential, driveshaft, T-case, transmission and finally engine compression. It seems odd you are feeling the problem coming from front of truck
If unable to narrow it down with further testing, you could try removing the rear driveshaft and run it in 4WD and see if it goes away, gets worse, etc. That way you will eliminate the rear driveline as possible cause.

 

Is this the truck in question? https://www.ford-trucks.com/forums/1...-dentside.html

The bouncing might have the do with the fact you have some rather old looking (and probably still bias ply) 16.5 tires on them cool old school slotted mags. The vibration could be the same issue caused by the tires or a drive shaft or front axle u joint. Or even some worn out shocks.

Also taking in to consideration is the fact it has "The steering was upgraded of the '78 style". according to your other thread. So was that done correctly, did it at least get a professional front end alignment once completed?

Bias ply tires can get flat spots over night and when first driven and can cause bouncing until they are warmed up and get back to being round. Insp the tires very well, you can even swap positions (front to rear, rear to front) and then take it for a test drive. Insp all the front end steering components, tie rod ends and all connections for being tight and in good service. Steering shaft connection points condition?

To insp the rear drive line u joints for play by hand, you need to have the drive line "unloaded" or have no pressure on it. So park the truck on a flat level spot, put it in neutral (not running) and chalk the wheels or put the brake/emergency on.

Get under there and now see if you can move the drive shaft (at each ujoint connection) up and down and then try twist the d/s while holding the yoke. Then insp the front. Since you were driving it in 2wd (confirmed yes?) the the front d/s should not be spinning and should not be causing any vibration.

Also insp the transfer case mounting points, motor mounts and transmission mounts.

 

You better get that U-joint replaced.

U-joints spinning at speed under a load will tend to self-center, load being just power to accelerate or just holding speed to go up grade, or even when slowing by down shifting will do it ... but U-joints spinning at speed with no load will vibrate if there is any excess wear or play in them, like would be the case if one just eases up off the throttle at speed, not applying power, not slowing either, but just enough throttle to match engine speed to road speed, that point between accelerate - decelerate.

Another thing is carrier support bearing, to check.

You feeling it from the front ... your hands on a steering wheel on a steering shaft that connects to a steering box mounted on the frame at the front of the truck. Frame connects to about whole truck.

 

Yes, that is the truck. The shocks were replaced when the motor was newly rebuilt. The gentleman I bought it for ran a Ford dealership and had a buddy with performance shop who did the motor and steering upgrade. I was going to take it to a shop and have the alignment checked and also mount make sure the tires were balanced. I am in the process of getting new wheels and tires as well. I will get in there and check the steering shaft later today. I did not check it with it being in neutral and not under load. I did notice some play coming from the driveshaft from the transmission to transfer case. Motor mounts are brand new. The t-case mounts could be upgraded as well as all the body mounts as they are worn out.

 

Check what kind (bias or radial) and what rim size are they. 16.5....they are probably a REAL old tire, meaning made years ago. That could be the problem.

 

They are 16.5 and radial tires

 

At least they are not bias ply tires.