Avant Severe Rear End Pulsation/Vibration

[vortex2450]

Hello everybody,

My A4 Quattro Avant with 215k miles has suffered a catastrophic failure somewhere in the rear end and I am not certain as to whether is in in the drive-train or the suspension.

Back story: The rear suspension has been slowing deteriorating over the past month to the point that the rear right side bushings will creak/squeak with any loading/unloading.

Last night as I was heading home I was decelerating to a stop sign when the car starting pulsating/vibrating loudly from the rear as I was coasting to a stop. Very noticeable. As if driving over a rumble strip at first. I slowly picked up speed to about 30mph were the vibration grew in intensity and severity until the point where I knew something was tragically wrong.


The best way I can describe the vibration is a pulsating up/down wobble that feels like a horribly out of round tire.

Thankfully I was 1/2 mile from home so I was able to limp back. I was able to drive the car up to 5mph with no ill effects. As soon as I breached that 5 mph limit the vibration would begin and climb with speed. Anything over 10mph and the rear end would begin bouncing.

Since I was planning a suspension rebuild soon anyway I have the vehicle up in the air now.

I have looked over the rear suspension and nothing appears drastically out of place. I pushed on the wheels before I took them off and they didn't give an inch..

Another note on the suspension, it is riding on Bilstein struts and H&R springs which have at least 40k miles (time since I bought the car with them installed) on them if not more. I have noticed a slow decrease in damping in the past 10k miles.

So, what in god's name is going on here?

Did the rear end diff fail?

Perhaps a CV joint failed?

Since the control arms are still intact perhaps the strut failed?

Tomorrow I am going to begin breaking everything apart so maybe I'll find something while I'm doing that.

So far my plan of attack:

- Remove rear spindles

- Remove rear axles

- Slowly disassemble the entire rear suspension (control arms/struts/springs)

- Remove rear diff and carrier

- Remove rear sub frame

- Replace all related bushings

- Replace struts/springs with coil overs

- Possibly replace both rear axles?

I'm not a fan of blindly throwing parts at a problem but this is all work I've been planning anyways however I still am not certain what is causing the issue here.

Any thoughts?

Thanks in advance!

[nemick]

Take a look at your center support on the prop shaft and also your forward diff mount

[Bordom]

You'll be in there for a few days at the very least. It sounds like you've had a catastrophic failure somewhere with a bushing or CV joint.

Get someone to watch as you drive around to see if anything shifts while moving.

Far fetched ideas would be:
1. All 4 subframe bushings miraculously failed at the same time causing your wobble/pulsing
2. All your CV joints decided to fail as well



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[vortex2450]

Take a look at your center support on the prop shaft and also your forward diff mount

The center support would be this guy correct?

I'm going to look it all over, the initial "signs" of failure per say sounded like a whirring before it become a pulsing bounce, so it's very possible there was a failure there.
There has been a small amount of play in the shifter which was pretty bad when I first bought the car but after new motor mounts and a snub mount it became much better.

You'll be in there for a few days at the very least. It sounds like you've had a catastrophic failure somewhere with a bushing or CV joint.

Get someone to watch as you drive around to see if anything shifts while moving.

Far fetched ideas would be:
1. All 4 subframe bushings miraculously failed at the same time causing your wobble/pulsing
2. All your CV joints decided to fail as well



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Yeah, I am afraid that is the case, luckily I have a lot of free time the next 2 weeks to dig into this but there goes my tax refund :|
I was under it for a few minutes today after I popped the wheels off, as I said there is nothing blatantly obvious but I haven't really put it under the microscope yet.

I am hoping it's suspension related but given the symptoms it was displaying in the 1/2 mile or so I drove it at varying speeds my instincts are leaning towards drivetrain.

The car is in the air now, and it's not going anywhere until I'm done with it.
I want to believe that some part of the suspension failed utterly as I really don't want to contend with a serious drive-train issue.
I probably won't have enough funds to buy a new diff or driveshaft at this point given I've already gone in for a suspenion rebuild but luckily I have 2 low mileage B5 V6's I can source from if need be.
Not ideal but for now it may be an option.

I will be getting to it first thing tomorrow morning and I will post pictures as I progress through the tear down.

Stay tuned for updates and possible carnage pictures.

[nemick]

The center driveshaft support in your link is the one I was talking about. I'll be changing mine soon too. Be sure to check it first, as it could very well be the cause of all your car's symptoms. A pain in the ass, I know, but part of me hopes it has failed as that would be among the cheapest fixes as long as you're doing it yourself.

[vortex2450]

The center driveshaft support in your link is the one I was talking about. I'll be changing mine soon too. Be sure to check it first, as it could very well be the cause of all your car's symptoms. A pain in the ass, I know, but part of me hopes it has failed as that would be among the cheapest fixes as long as you're doing it yourself.

Well I started it up in the air with the tires off (I know, not a good idea but I needed a diagnosis) and the problem is ABSOLUTELY drive-train related.
Faint vibration at idle and it shakes the whole car at 1500rpm which is hard as I will willing to push it.
I did not see anything jostling about but the center/rear diff mount is definitely shot and a lot of the other bushings aren't far behind.

From what I understand even a slight misalignment in the Quattro system can have severe side effects so I am hoping you are right on the drive shaft support.

I'm going to tearing the exhaust system apart soon and get a better view of everything.

[Bordom]

You're looking at a rear end refresh of all bushings ($400 for all if I remember correctly) and a new rear diff mounts. (ECS one is $150 and much cheaper than OEM)

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[nemick]

Once you remove the heat shield from under the drive shaft, you should be able to see/feel if that is at the root of your problem. Typically, the rubber just gives up and lets the shaft flop about to a varying degree

[OMEGA SUPREME]

I had my center support bearing go out. It didn't vibrate that badly. I am thinking rear diff. Have someone look under the car while in gear. Not under it.

[nemick]

What were your symptoms when your's went out?

[vortex2450]

Once you remove the heat shield from under the drive shaft, you should be able to see/feel if that is at the root of your problem. Typically, the rubber just gives up and lets the shaft flop about to a varying degree

I'm trying to get there however I am currently raging an epic battle against the top downpipe flange nut on the passenger side. This is almost as bad as replacing the motor mount.

I was able to grab the propeller shaft at the end closest to the transmission and I can jostle it a fair amount.

Had to take a breather after I busted my knuckle, going to have to get the exhaust fully off before I end my day.

I had my center support bearing go out. It didn't vibrate that badly. I am thinking rear diff. Have someone look under the car while in gear. Not under it.

This might be the case, what is silly is I tried changing the fluid 3k miles ago to avoid this from happening but I was aptly defeated my the fill nut ended up rounding out not only the nut but my tool as well.

I was planning to change it when I changed the rear sub frame mounts....

I did this, I had my dad slowly feed out the clutch while I peered under and I can not seem to pinpoint any obvious movement in the rear drivetrain but since I couldn't see the drive shaft I can't be certain what is going on.

I will update soon. The only upside is that this happened while I was home and not while I was a few states away on a job... That's a scary thought.

[Bordom]

Wait, so you emptied the rear diff and didn't refill it after realizing you stripped the shit outta the fill plug?

I would 100% say it's the diff then

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[nemick]

I'm hoping he tried to remove the fill plug before draining and failed

[OMEGA SUPREME]

What were your symptoms when your's went out?

Slight vibration at certain rpms. Most notable 2200-3400.

[vortex2450]

I'm hoping he tried to remove the fill plug before draining and failed

What he said, of course I didn't drive with the diff empty. I don't claim to be a genius but give me a little credit!

Got the exhaust off and proceeded to whack my head against the lower control arm. Done for now.

More results tomorrow.

THanks for tuning in guys!

[nemick]

I feel like an expectant father here. Need an update, lol

[vortex2450]

Driveshaft is out, doesn't seem to be broken. U joint is solid and the CV joints are tight.

Digging into the rear end now. Hoping that the issue is in one of the rear axles (ie CV joint).

[nemick]

Fingers crossed

[nemick]

Anything?

[vortex2450]

Anything?

Getting rained out, wish I had a garage right about now.

[nemick]

Oh man, I didn't realize you were doing all this Al Fresco. I feel for you. I hope the weather improves

[vortex2450]

Oh man, I didn't realize you were doing all this Al Fresco. I feel for you. I hope the weather improves

Thanks man, me too!

I got myself in a tight spot now. I ordered rear axles from ECS tuning as well as the rear diff mount and the support bearing since I'm at it anyways.
Don't like not knowing what the issue is yet, between work stuff drawing my attention away and the rain this is starting to draw out.

[vortex2450]

Update. Pretty damn sure it is the driveshaft, the u-joint is seized on 1 axis. Debating whether to order one and wait or to pull one from a JY.

Still going to tear into the rear end but at least I have a culprit.

[nemick]

I'm not positive, but I don't think the UJ is even serviceable. Might need a replacement drive shaft from a JY

[vortex2450]

I'm not positive, but I don't think the UJ is even serviceable. Might need a replacement drive shaft from a JY

Definitely isn't. Going to pull one tomorrow, meanwhile the rear subframe is dug in like a tick. Brake lines, parking cable, ABS sensors. I'm getting my butt whooped.

[Bordom]

Definitely isn't. Going to pull one tomorrow, meanwhile the rear subframe is dug in like a tick. Brake lines, parking cable, ABS sensors. I'm getting my butt whooped.

Yeah man. You have to mangle the mounting points for the parking brake in the lower control arm. It's just a plastic clip you break off and then chip off some of the protective covering to slip it out.

You'll be in there a while. Especially with the corrosion going on under there

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[vortex2450]

The left side is giving me hell. I broke the s**t out of the ABS sensor, the tie rod nut is seized and the joint is spinning and the lower bolt won't budge. Right side was butter so I'm failing to understand why the left is the complete opposite.

[nemick]

The right side might have been apart more recently. Hang in there bud

[Bordom]

The left side is giving me hell. I broke the s**t out of the ABS sensor, the tie rod nut is seized and the joint is spinning and the lower bolt won't budge. Right side was butter so I'm failing to understand why the left is the complete opposite.

Undo the 4 bolts on each side holding the strut mounts to the car. Undo the brake lines and parking brake mounts then slide the parking brake out of the lower control arms.

Once everything is clear you should be able to do the subframe. With it out of the car you'll have much more room to work

Everything graciously mispelled by Android

[nemick]

Any progress?

[vortex2450]

Any progress?

Subrame is out=. I was going to go pull a driveshaft but the rain is torrential here.

[nemick]

Good job on getting the subframe out. Have you found anything other than the driveshaft UJ, yet?

[vortex2450]

Good job on getting the subframe out. Have you found anything other than the driveshaft UJ, yet?

It wasn't perfect, the left side strut bolt is totally seized in the bottom of the strut, I am trying to get it out without compromising the bushing on the strut.
The sway bar bushings are not tight with the sway bar and I suspect that was the source of a clunk in the rear. Also, the strut mounts are definitely corroded along the inside seems.

Almost time to press out the control arm bushings.

[nemick]

It wasn't perfect, the left side strut bolt is totally seized in the bottom of the strut, I am trying to get it out without compromising the bushing on the strut.
The sway bar bushings are not tight with the sway bar and I suspect that was the source of a clunk in the rear. Also, the strut mounts are definitely corroded along the inside seems.

Almost time to press out the control arm bushings.

IF you decide to press out and replace the rear control arm bushings, would you mind making a note of what size press tools you use? I'll be replacing one in the summer and any info would be a great help.

[vortex2450]

IF you decide to press out and replace the rear control arm bushings, would you mind making a note of what size press tools you use? I'll be replacing one in the summer and any info would be a great help.

I
lle be pressing them all out, I can shoot pictures and note all the relevant sizes and post them here if that would be helpful.

[nemick]

I
lle be pressing them all out, I can shoot pictures and note all the relevant sizes and post them here if that would be helpful.

That would be very helpful for me personally and I imagine it would help others too as our cars age.

[nemick]

Just looking for an update on this job

[vortex2450]

Hey guys, this has been a total nightmare. I have the rear end 98% together now, just trying to source a few bits of hardware. The local JY only had one 5 speed A4 so I was able to source a used drive shaft. I took the CV joints off, disassembled them and refurbished them as well as filling in the carrier bearing bushing with a flexible adhesive.

Low and behold during the time that the car was sitting the crankshaft position sensor said it's final prayers and failed utterly, the car failed to start before a few weeks ago and I didn't place the issue until now. I haven't checked codes yet as I cannot locate my scanner tool but the tach does not register at all when cranking. The fuel pump primes when I key on so I believe that is good.

I rigged up a wooden jog to install the driveshaft and I am getting ready to put that in and get it centered up. After that all I need to do is re install the exhaust and hopefully manipulate into a high position than how it was ( hanging low and scraping everything).

I took a fair amount of pictures but not comprehensively because they were many points were my hands were either too greasy or I was too damn mad to care about pictures.

However, once I have this back together and have the free time I will likely put together a few DIY guides (rear suspension refurb, driveshaft refurb/DIY guide, etc).

I wasn't going to pay the local auto stores $100+ for a third rate sensor so I ordered a BOSCH sensor from amazon, it will be here tomorrow. Then it's just a matter of getting an alignment.

[nemick]

Sounds like you're almost there. Hang in there.

[vortex2450]

Sounds like you're almost there. Hang in there.

It's all together now. I get to relax now since all I have to do is throw a new crank position sensor in (hopefully).

I am hoping I got the drive shaft in straight enough to be happy.
I made a mounting jig using a 3' 1x6 and 4 peices of the same 1x6" with 2 1/2" holes drilled into them and then tightly zip tied to the board.
After I mounted the shaft I made a test jig to for straightness and rotated the driveshaft while adjusting the carrier position until the driveshaft rode without pushing any part of the test fit jig off ( 4 x 2" peices of 1x1 mounted at equidistant length on a 3' 1x1" ).

Just to be sure I then measured the difference in distance between the knuckles of each side of the shafts at the u-joint and adjusted a smidgen to get them totally even.



Truth be told now that it is 99% done I feel accomplished. This was by far the most frustrating and involved auto repair I've had to make to date, and I've swapped engines and transmissions multiple times.

When I get around to making a thread on all the work I did I'll post the link(s) here.