Hello,

I need some help troubleshooting what would be causing a rather significant vibration. I'm quite handy around vehicles but this one is stumping me,

Here are the details:
Only is noticeable above 45 mph
Only happens when you take foot off gas, goes away as soon as you touch gas again (so happens below 1700 rpm)
At 60 mph it is a very fast vibration, very pronounced
You don't feel it in the steering wheel; at 60 mph I feel it in the seat
Tranny shifts smooth, no clunking or whining and fluid looks:smells ok

I thought maybe motor mount but they look ok, going to check tranny mount today.
If it's suspension then I would expect to feel it in the steering wheel but I'm also not eliminating it.

1999 f150 4.6l v8 2 wheel drive extended cab (my summer trailer hauler, just came out of winter storage)

Any thoughts/diagnosis suggestions are appreciated.

Gary

 

Check the u-joints and driveshaft yokes.

 

Yep, I agree with glc.

 

Ok, thank you very much. I did look at them and nothing obvious. Can't recall last time I greased them so did that (aftermarket ujoints) just now. Will test that out and if no luck will drop driveshaft and check in detail. I wonder if a weight fell off?!

 

My slip yoke and flange yoke had to be replaced due to slop developing in the u-joint cup seats. Yokes are expensive and hard to find, rebuilt driveshafts for a manual trans standard cab short bed 2wd are impossible to find. I found a driveshaft in a boneyard that was dented but the yokes were good, cost me $25. Cleaned up the yokes and swapped them over to my shaft.

 

generally speaking:
if you feel it in the steering wheel, its front axle related
if you feel it in the floor/seat its likely rear axle related

 

Issue resolved, the rear yoke was missing one bolt and a second one was halfway out; the remaining 2 I could remove with my fingers. The vibration was caused by the yoke slamming against the halfway out bolt. Got 4 new bolts and installed them ensuring they were torqued to 60 ft lbs.

Thinking back to last fall, I recall my friend reinstalled the driveshaft after we did some work under the box; I think he used the German method of torquing - gutntite - grrr

Thanks everyone for your help.
Gary

 

Glad you solved the problem! And it only cost 4 bolts! And thanks for letting us know.

 

That would have made quite a lot of racket if it came off going down the road