Weird Vibration - doesn't fit the models - SOLVED!!

I've read a bunch of posts, crawled around under the truck, driven with a mechanic, and am still scratching my head. Mechanic seemed stumped but wants me to bring it in for 'diagnostics'.

Here's what I know:

Only happens when going downhill and intensity changes with road grade.
Only happens when engine braking and 'sometimes' with very light throttle.
Goes away immediately when accelerator is pushed and drive line tightens up.
Does it in neutral as well, but will disappear *if* put back in drive and accelerator is pushed.
Does it in both 2hi and 4hi.
Changes with speed - that's weird, since that would seem like axle trouble and not drive line
U-joints look okay, but drive line has not yet been pulled
Truck seems to have more axle wrap than other Tacomas I've owned.

It's weird that the frequency of the vibration changes with speed, yet disappears when the accelerator is pushed. Doesn't seem to be RPM dependent, just needs tension on the driveline to go away.

I'm reluctant to take it in to shop for diagnostics without a good hunch as to what it can be. That scenario always seems to generate a bill that adds up fast and leaves little choice.
He wants to check over the front end and then pull the drive line. But he didn't really have an idea as to what it might be and called it 'weird'.

 

center carrier bearing? any movement in it?

 

There's a little play in the center bearing, but nothing too excessive. The mechanic agreed with that. Still, it might be where I start. I just don't want to start throwing money at it, hoping to stumble on the right solution. OE parts add up so quick. And I'm not entirely sure it's the driveline, it just seems the most likely candidate. I never have a straightforward problem on my truck.

 

something that can be hard to find and seam "hidden" is the front end control arm bushings let the wheels flare out under heavy braking

test by holding front brakes and forcing the truck forward with the gas, then set parking brake and turn off engine while still in gear and get out and measure for tire movement. this tests the rear side control arm bushings, to test the front side control arm bushings repeat the process moving in reverse this time

 

SOLVED!!!!

It was getting worse and seeming unsafe to drive. I stopped by a shop that I had heard good things about and after a quick drive and 5 minute inspection the mechanic found the problem.

The nut that fastens the yoke (the one by the cardan joint) was loose. It seems the prior owner had replaced the center support bearing and didn't tighten the nut properly so that it would lock, and it had come loose over time. The looseness was causing the drive line angles to change under load, making it vibrate/shake, and giving me bad axle wrap.

A LOOSE F-ING NUT!!!! The driveline was rearing to fall out on me.

As to why it presented the way it did:

It would show up during engine braking when going down hill because the incline of the road and lack of throttle would allow the driveline to attain a bad angle and present. Opening the throttle would change the angle of the driveline as it went under load, putting it back in-line. Tightening the bolt removed the play in the driveline there and now it holds the right angle at all times.

 

On a side note, how do you edit the title of a post? I want to add the word 'solved'. I can't figure it out on this updated forum. When I click 'edit' post, and further click 'more options', I don't see a way to get to the title.

 

above your first post, on the right hand side you will see "Thread Options." click the down arrow and click on "Edit Title"

 

Yeah, you fixed it without draggin the truck to a dozen "specialists"! This is the first time I heard of a vibration similar to mine, but I'm told it's because my truck was hit hard in an earlier life and has something "unhappy" out of line in driveline. Was just wondering (forgive the newbie-ness) exactly where the bolt that you mentioned is? It's worth me checking. Drive shaft was pulled, center bearing replaced, tranny checked, everything was fine. Still get the vibration. But get this, the leaf springs had been altered by PO just to make the truck level. Could this cause "axle wrap"? Anyway, I had posted a thread about my vibration, it only happens in the speed range 22-26 mph! I've all but given up hope, nobody can isolate it. But I'm glad that you nailed your problem. Dang it's a bugger.

 

Truckrider- he's talking about the big nut that holds the flange on right behind the center bearing, so if you replaced your center bearing this nut would have been removed and reinstalled so is probably not your problem unless the nut was loose before and whoever did the bearing still didn't tighten it proper. Have you checked u-joints? The symptoms described in this thread are also the same symptoms of a bad u-joint.

OP- did you happen to get a look at the splines in the flange before the nut was tightened? I had the same problem and just tightened the nut down and drove for another year or so then when doing my clutch noticed that the flange would slip/twist about 1/4" on the splines so I pulled the flange off and the splines in the flange were about gone. Driveshaft splines still looked good but the flange splines were just little peaks and I figured it was when the nut was loose it wore out the splines. Went down to a driveline shop and had a new flange the next day for about $60

 

Oh I see now, and yes I had the center bearing replaced, so that nut is probably tight. I'll have a look though, and btw it's worth checking the splines in the flange. Thanks, cause I pretty much checked everything else. Strangest thing is that the vibe happens 22-26mph even rolling with engine off and tranny in neutral.

 

of course it was something insanely easy. thanks.

 

I didn't actually see the splines, but the mechanic said there was no apparent damage he could see. Even so, I would imagine there is some wear. It had been 10 months since I noticed the very slightest vibe, and I put a lot of miles on it during that time, with it slowly getting worse. I'll look into it when I'm under there again.