Help diagnosing what appears to be a bad/seized differential
 

Hey FT86 Fam,

My 2013 BRZ started making funny noises while pulling into the driveway one night... grinding, clunking, and vibrations were all included.

The noise and vibrations happen when I have the car in gear, rev it up, and then shortly after the clutch engages the fly wheel, putting power down to the drivetrain, all hell breaks loose.

I jack it up and take this video: https://www.youtube.com/shorts/O8Ci_8FnjIA

Based on a friends recommendation, I replace the driveshaft. This yields no change.

I then do some more digging around and see one of my axles has a huge tear in the CV boot. I go ahead and replace both rear axles. I also changed the diff fluid at this time, and see nothing out of the ordinary in terms of extra specs of metal coming out. Still, same problem after both axles are installed.

I had a friend lay down while my car is on jack stands, and as I release the clutch to have the drivetrain engaged, he noticed that the differential is hopping/jumping up and down, which is what is causing my car to feel like it's hopping.

I have driven the car pretty hard, and I have had a cusco differential brace installed for well over 50k miles.

I am down to replace the diff, I just want to make sure I am not missing anything, as the diff is a pretty large undertaking. Am I safe to assume that something is bad in the diff case? If i go about buying a new/used pumpkin, should I remove the cusco brace?

Any help is greatly appreciated.

Blake

UPDATED Videos below!

https://youtu.be/QOLjg3dZTuM

https://youtu.be/Ni2vzktrMlM

If you get the rear wheels off the ground and put the car in neutral and turn one wheel, does it turn smoothly?

if the diff is moving around abnormally, I'd replace and/or reinforce the diff mounts, trans mounts etc.

I'd also be looking at the clutch etc

This ^ is also the first thing I'd check. Also rotate the drive shaft by hand to see if anything catches.

Have you inspected all of the diff mounting points to ensure the bolts are tight and the subframe isn't cracked? The diff shouldn't be allowed to hop/jump up and down.

Just to be clear, the noise only happens once you've released the clutch and not before? The sound is very similar to when my engine developed rod knock.

Post pics of rear cradle and front diff bushings. May need to remove that brace you have to get a proper look.

A long time ago in another galaxy, another junior mechanic asked me about an MG-B that came in the shop that was from a customer who asked for me. The service manager yanked that ticket and gave it to someone else which pissed me off royal. I had a full slate of work, so I got over it. The other mechanic who got the job asked what is suspected, so I gave him a snarky flip answer, "I don't know, but maybe the differential has a bad bearing." I still feel bad about it fifty years later as it turned out to be what I would have checked for if it were my job: an easy fix... bad rear prop shaft u-joint. I have overhauled my share of transmissions, overdrives and other drive-line components and you learn to eliminate the the easy stuff first. Assuming that the drive shaft and axles have been checked out, isolate the differential and check it. Aside from normal wear and tear from miles, the most common cause for differential failure in my experience is what comes after I ask a question of the owner, "What did you hit?" They never volunteer that bit of information when they bring the car in. The Torsen differential in the BRZ/86 I would not call it a true limited slip type as it is more of a torque bias gizmo. If one wheel has no traction at all, things go badly. With both wheels on the ground, it does a good job of putting the torque in the right place. Off road or ralley I would go with something with clutches in it. The design in the Torsen that makes it quiet in turns is also a major weakness. In my opinion, a sudden shock such as a pothole during a turn could mess up the little geerzmos in the diff. Can't tell too much from the video, but I would take a hard look at u-joints and then move on to the differential. Rebuild only if you can't find a replacement.

Both gear type & clutch type diffs have own sets of pros & cons. Noise of operation being only one of them (i'm guessing that OEMs lean to torsen more for reasons like ease of maintenance too, +some of drawbacks they compensate using electronic nannies operating brakes, which is much simpler & cheaper for them).
Meanwhile, saw interesting video along the line "why not have both" :)
https://www.youtube.com/watch?v=XuqdWkYiO-M

Alright, first off, THANK YOU to all that posted!

The is on jacks, and i tried turning the wheels to be met with little to no resistance. Just sounds like the diff is doing its normal thing!

I realized that the video i took is of a terrible angle, so I decided to help you guys out, and myself, and take a few new videos, one with the Cusco diff brace on, and another with it off.

Take a look for your self. It look likes the driver side diff bushing is shot?

https://youtu.be/QOLjg3dZTuM

https://youtu.be/Ni2vzktrMlM

Nope diff bushing is doing what is suppose to do. It is time to start doing exploratory on it. Pull half shafts and check, then pull drive shaft and check. If nothing turn diff by hand and check. If nothing open up diff and check. If not there time to check transmission.

I have lifted the rear of the car up, taken the drift shaft off, and found that the car ran fine. Problem starts when the drive shaft is bolted to the diff.

I have replaced both rear axles, so confident they are not the culprit.

Differential looking more and more likely.

I have had to replace the front and back right side coils several times from hitting curbs and pot holes. The roads are terrible in my area, and i diddd have to learn how to drift some how many many moons ago. I suspect this level of abuse has taken it's toll on the diff! Good call pointing out the beating that I have put the car through in it's years. I don't drive the car easy, as it's most fun when it's right on the tail edge of control.

How do the bushings on your driveshaft look? When I had mine replaced with the metal torque solution ones (a mistake), the stock rubber ones were super deformed. Not sure that could cause a noise though.