The truck shakes at 47-53mph and at 67-74mph.
Greetings jakeboyce33,
Tough vibration issues that resist normal corrections (tire rebalancing, shock replacement, etc) can
be fixed, but it will require careful observation by you, for both when the vibration is present *and* absent.
Good on you for observing that the vibration comes, goes, and returns. More on this in a bit.
When looking for the source of a vibration, they can be divided into 3 main categories. Figuring out which
category the vibration falls into will help tighten the problem description, lessen the real estate we're focused on,
and get us barking up the right tree so to speak. Here are the categories:
* Vibration is engine (and/or accessory) speed related
* Vibration is driveshaft/road speed related (with no torque / acceleration / deceleration)
* Vibration is driveshaft/road speed AND torque related (vibration worsens with acceleration)
NOTE: There can be some of each of the 3 categories above. For example, a small vibration in
the serpentine belt driven accessories and a small imbalance in the driveshaft can add (reinforce)
and subtract (cancel) each other, with this being rpm / frequency-dependent.
In order to keep this reply from becoming too long to read, I'd like you to try the following and
report back with your observations:
Q1) Drive through the vibration speeds as you normally would in order to establish a baseline.
Now accelerate to just above the highest speed, and then put the truck in Neutral and allow
the engine to idle while the truck coasts back down to ~40 mph.
Q2) When the truck coasted through the vibration speeds, were the vibrations still present, or
were they much less?
Q3) Does accelerating through the vibration speeds make the problem worse, or no change?
Q4) Assuming a gear change, is the engine rpm the same at the 47-53 and 67-74, or not?
Q5) Is your truck lifted? If so, what are the angles that the u-joints in your driveshafts are
running?
Q6) Has the truck recently been in an accident?
Q7) Has there been any catastrophic powertrain failure that has been repaired?
Q8) Has the truck ever driven smoothly at these speeds during your ownership? If so,
how long ago was this? And what exactly has been changed since then?
****
Quick personal story. I had a 3 year old car that ran well without any vibration at any speed.
On a 2-3 upshift I grenaded the rear end. So I purchased a brand new stronger rear end
from the motorsports division of Brand F. Subsequent to the replacement I now had a
vibration at 75+ that resisted repair. This was pre-internet, so all I had was the factory
service manuals.
After spending a lot of time & effort trying to troubleshoot the replaced differential, it turned
out that when I grenaded the previous differential I also twisted the driveshaft a little. (maybe 10°)
It wasn't easy to see, but when placed on a long, straight workbench surface it was obvious that
the front & rear u-joints were no longer working in the same plane. (The driveshaft wouldn't sit
flat on both u-joints, but instead rocked between the two.)
Replacing the driveshaft cured the vibration. (And the lesson that it takes a long time to fix something
that isn't broken was reinforced to yours truly yet again. :0)
I also had Goodyear Ultra terrain tires put on in before winter. I've had them re-balanced and the rear end still shakes. Rims are stock size and still in good shape. Every once in a while while going 70mph it's as smooth as a babies bottom going down the road but the bounce eventually comes back. Wheels are tight, I've swapped the front tires to the rear on both sides, on the side it shakes the most I tried a different drum and it still bounced/shook. It's a solid rear axel back there. I'm not sure what else to try at this point? Any advice is appreciated, this is driving me nuts!
OK, this is good data. Life is easy when a vibration is only being generated from one root cause.
It is a lot harder when 2 small vibrations are reinforcing and cancelling each other, depending upon
the speed of the engine / accessories vis-a-vis driveshaft/road speed, and torque as a third variable.
****
I've always had an interest in automotive NHV (Noise, Harshness, & Vibration) ...and I value something
like an old GMT400 chore truck that runs very smoothly at all combinations of speeds & loads.
For me, few things are more off-putting than a vibration at speed. (!) The good news is that GM Factory
Service Manuals have one of the very best Vibration Diagnosis sections of anyone, anywhere. I've attached
just a couple of pages to pique your interest. If you haven't already done so, then go to this forum's FSM
cache of links that allow you to download these manuals. (
88+ Factory Service Manuals)
I guarantee that if you download the applicable manuals for your vehicle and study the Vibration section,
between this and the combined experience in this forum that you will be able to track down & eliminate
those vibrations.
Please evaluate your truck based upon the questions above and reply to this thread with the requested
info. This way we can kick this around and come up with some reasoned suggestions. Sharp photos &
even video are very helpful for those of us trying to assist remotely.
Happy vibration Hunting --