Vibration when backing off gas

[Sweet Lou]

Howdy all! My car is a 2008 SEL AWD. I will try to be as exact as possible with my symptoms. 

 

Over the last two, maybe three weeks, I've noticed some vibrations. Not just in the wheel, but like the entire car was doing it. Felt in the seat, in the floor, etc. In preparation for winter, I had a complete set of new tires installed for days ago. No change to the vibrations. I did some research and found what it possibly could have been caused by. I had the stock 3 piece drive shaft with the two carrier bearings. Both bearings had degraded rubber and were allowing a lot of movement of the shaft. After discovering that they could not be replaced, I ordered a 2 piece aftermarket replacement drive shaft from O'Reillys. I went for a short drive and everything seemed improved in terms of vibration. Turns out I didn't go fast enough for my test, apparently. 

 

Under load (accelerating, climbing a hill, etc) at any speed, the vibration is completely absent. At around 45mph, if I let off the gas to coast or if it's a flat stretch that doesn't require much power to maintain speed, the car vibrates again. As bad, possibly worse, than before. Higher speed just means more vibration. 

 

In an attempt to narrow it down, I looked at rear drive components. Both CV axles appear free from damage or play. Replaced rear driver side wheel bearing recently, both rear bearings are tight. Grabbed drive shaft, no discernable play or slop at the PTU and rear diff connection. I do have some visible signs of leakage from the CV axle seals at the rear diff as well as from the plug area. Removed plug, fluid seems full, but does have that wonderful smell of degraded and overheated oil. 

 

The only other thing I did involved the two nuts holding the carrier bearing. When I completed install of the new shaft yesterday, I essentially tightened them as far as they would go. I used the original washers, rubber bushings, and nuts. After I inspected everything today, I loosened the two nuts some. I basically tightened them just to the point where the rubber bushings stopped spinning when I tried to do so by hand. After another test drive at higher speed, the vibration seems reduced. It still comes on almost exclusively when I let off the gas. Unlike before, it doesn't really start until I hit the 55-60mph range. I let off at 60, it vibrates some, but not as bad as before, and disappears once my speed reduces to about 55.

 

As it sits, I'm considering just some new bushings and such for the carrier bearing and leaving as much play as I can. I'm figuring that having it too tight allowed an unacceptable amount of vibration to the car. My other thought is to return to the tire shop and have them double check balance on them. I think if it were a tire or two it would be a more constant issue, but suppose it's worth an ask. 

 

Any other suggestions? Maybe try and 180 the shaft at the rear, see if it balances better? Thanks!

[DILLARD000]

Could be the PTU failing, definitely a NotoriousFailureItem.

Miles on the vehicle?  How long since PTU GearOil was renewed?

Personal preference is at least every 16kMiles\2Yrs with 20oz of 75w140.FullSyn AmsOil or RedLine.

Edited October 16, 2023 by DILLARD000

[enigma-2]

I remember reading where someone else had a similar problem, few years ago. Turned out the new shafts changed the angle going unto the RDU.

 

Also a worn pinion gear or bearing can cause this. (Most common cause is the drive shaft bearings as you already deduced.)

 

Seen a YouTube video sometime back (don't  remember the title) where the mechanic placed a vibration sending unit (magnetic) on or near various parts of the drive train to narrow down the actual location of the vibration. Had to place it on several locations but eventually narrowed it down.