89 BII Vibration

[86isuzu]

1989 Bronco II, stock.

I don't drive this truck much, it sits a good chunk of the time. But I did drive it a few times this year and everything functioned as expected. However, I drove it this past Saturday and it started vibrating like I was driving down a rumble strip whenever I was decelerating - doesn't matter whether I'm coasting or braking - until about 20 mph or so and then it's back to normal. It is perfectly smooth when accelerating. Doesn't matter if I am going straight or turning. I replaced the driveshaft u-joints last fall. Tires are near the end of their life, but still have legal tread.

I am truly puzzled.

 

[JerryC]

Do you hear any clunking? Does it just vibrate or do you feel like it is going up and down?

Check your lug nuts.

 

[alwaysFlOoReD]

4x4 or 2wd? Auto or manual?

 

[86isuzu]

Do you hear any clunking? Does it just vibrate or do you feel like it is going up and down?

Check your lug nuts.

No clunking. Just a harmonic vibration. At first I thought maybe it was a flat spot on my tires. But the truck was only sitting about 2 weeks since I last drove it. And after about 45 mins on the road, it was still there.
Lugs are tight, I did double check them.

Manual tranny, manual t-case, & manual hubs.

My co-worker, a rather experienced mechanic, seems to think it might be caused by some mild axle wrap. I suppose its possible - it only occurs while decelerating. Sure, its got saggy BII springs, but its a pretty tired 2.9 as well.

 

[JerryC]

Does the vibration continue for as long as you decelerate or does it just happen for a short time?

 

[86isuzu]

Mostly. It stops right around 20 mph. It starts as soon as I let off the gas otherwise. Coasting in gear, neutral, or braking and then the vibrating starts. Turning one way or the other (as best as I can) doesn't alleviate anything.
There's no unusual noises (other than the noise of the truck vibrating). No clunking, or groaning.

 

[JerryC]

It's hard to say from that. You're probably just going to have check everything, suspension, tires, driveshaft, etc...
Where do you feel it strongest? Steering gear and front suspension should come through the steering wheel. If the steering wheel is not vibrating then you can probably rule out most of the front of the truck.
If you feel it through the shifter, that would point to driveline.

 

[86isuzu]

Ok. Thanks.
I'll have to do a thorough road test and double check.

 

[86isuzu]

It's hard to say from that. You're probably just going to have check everything, suspension, tires, driveshaft, etc...
Where do you feel it strongest? Steering gear and front suspension should come through the steering wheel. If the steering wheel is not vibrating then you can probably rule out most of the front of the truck.
If you feel it through the shifter, that would point to driveline.

I would say strongest from the shifter. A little bit from the steering wheel, but I don't think that's the source.
Time to look at the driveshaft again. Double check the condition of those u-joints I replaced last fall.

 

[wend3lrt]

Had the same vibration problem and it was because of a bad pinion yoke.

 

[86isuzu]

Solved.
The driveshaft bolts worked themselves loose. All good now.

 

[alwaysFlOoReD]

Thanks for posting the fix.