Peanu_Keeyes GRM+ Memberand New Reader
4/15/24 11:01 p.m.

POP! The drain plug shot out from the diff under extreme pressure, spraying hot gear oil in my face, eyes, exhaust, driveway…. everywhere. Like a sun baked garden hose set to Jet.

So I picked up a 4.3 Torsen for my NA late last year. Carrier was open and I was able to check that everything was orthodox with the gears and bearings. I replaced the axle seals with some RockAuto cheapies, buttoned it up and set it aside for four months while I rebuilt the engine. On Easter Sunday, I started it up and bed the pistons rings, but the car suddenly had taken on a new character. No, not speed. Gear wail.

I had installed some polyurethane differential mounts in the new to me 4.3 Torsen — could this really be the cause? I was suddenly a dive-bomber with Formula 1-like gear whine. I actually loved it but this was not the kind of NVH I was told to expect and so the dread set in of having to drop the pumpkin to adjust the lash and inspect the bearings. I checked the fluid.. it looked clean, and I’d now changed it twice in two weeks. I ordered a Blackstone lab kit and decided to change the oil again this afternoon, only this time I didn’t let the car sit long after my drive before opening the plug. 

With one more turn left on the drain plug —POP! 

After washing my eyes out for several minutes, I started looking this phenomenon up online but couldn’t find much in the moment. Then common sense prevailed and I realized there was no air being allowed to escape — the breather valve on top of the diff was jammed, welded shut with rust. After battling the vent out, I drove the car again with fresh fluid. 50% quieter INSTANTLY.

How I missed this when I had everything on the bench, I will never know. It’s now so much more livable for my drive to and from the track. It feels like it drives smoother too, though I’m sure that is placebo. The most amazing part of the story is the RockAuto axle seals which are apparently pretty solid!

So there you have it. Don't forget to check those relief valves...

Floating Doc (Forum Supporter)
Floating Doc (Forum Supporter) GRM+ Memberand UltimaDork
4/16/24 12:07 a.m.

I'm glad your eyes are okay. 

Hoppps Reader
4/16/24 7:40 a.m.

This is alarming. Glad you're okay....but now I'm worried about my rusty NA. Where exactly is the breather? And is it accessible from under the car?

Edit: a quick google search and I found this video on how to replace it. Considering the rest of the car is a rust bucket, I'll be adding this to the list before running the car again. Thanks for the PSA!

Peanu_Keeyes GRM+ Memberand New Reader
4/16/24 8:54 a.m.

I hope you can still put a wrench on yours, mine was absolutely buggered. Very frustrating since there's not a spot of rust anywhere else on the diff that I can see. 

NickD MegaDork
4/16/24 9:41 a.m.

I also had an issue with the breather on my '94-'97 rear differential. I had issues where it kept wanting to push fluid out past the pinion seal, and then I realized that when I removed the fill plug there would be a whoosh like it was relieving pressure. Ended up having to drop the differential, one, to replace that breather, and, two, to replace the axle and pinion seals.

Peanu_Keeyes GRM+ Memberand New Reader
4/16/24 4:03 p.m.

In reply to NickD :

Did you notice any change in how it sounded before / after? I'm still of the opinion that something else is misaligned in my diff because it's still technically too loud but my goodness is it quieter than it was. 

NickD MegaDork
4/16/24 4:51 p.m.

In reply to Peanu_Keeyes :

Mine was regeared with a 3.63:1, and despite all the measuring and adjusting and double-checking, it has always had gear whine. Then after fixing the non-breathing breather over the winter, it did one autocross and the pinion bearings died. Then I had another shop overhaul it, and it's still noisy. So I honestly don't know.

ShinnyGroove (Forum Supporter)
ShinnyGroove (Forum Supporter) Dork
4/16/24 5:10 p.m.

This is a known failure mode- changing that breather valve is SOP any time a Miata diff is off the car. Glad your eyes are ok!

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