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      08-20-2021, 07:21 PM   #1
southpaw92
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Lightbulb F30 leaky differential mount - Please educate me

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Hi everyone, CPO warranty of my 2017 330i RWD is due to expire soon and I thought of getting it checkout by a indy shop, for finding any potential repairs.

The shop informed me that my "front" left and right differential mounts are leaking.

I am not an enlightened person when it comes to cars, so I came back home and read up what "differential" is. Few internet search later I learned that RWD cars only have rear differential :| .

Can someone enlighten me whether F30 RWD has anything called a "front" differential.
Assuming it is the rear differential that's leaking, it will help if someone can explain to me the importance of repairing this and how long can I delay the repair. Is this something that is covered under CPO warranty?

Just thought of checking in with the forum before I talk to dealership next week. Thanks!
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      08-20-2021, 07:39 PM   #2
Wgosma
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The FRONT suspension has two sets of fluid-filed rubber bushings - Thrust Arms and Control Arms. Over time and use the rubber housings of these bushings will deteriorate, crack and the fluid fill inside will begin to leak out. At that point the part definitely requires replacement.

My guess is this is what your shop/mechanic is referring to. As you know the car's differential is located at the rear axle, it is filled with a relatively heavy gear oil, the differential case is a multi-piece assembly, the parts are bolted together w/gasket in between....it's possible that a gasket could be leaking a bit of gear oil.

Revisit this issue with you shop for clarification of the issue.
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      08-20-2021, 07:47 PM   #3
southpaw92
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Attaching pictures I got from the shop. Hope that helps explain if its rear differential.
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      08-20-2021, 08:15 PM   #4
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That is definitely the rear diff, and looks like the bushings are shot. PITA to replace..
I think your shop meant the front bushings on your differential
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      08-20-2021, 08:26 PM   #5
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Quote:
Originally Posted by im_an_alien View Post
That is definitely the rear diff, and looks like the bushings are shot. PITA to replace..
I think your shop meant the front bushings on your differential
I just looked at the R&R procedure....it's indeed a bit of work, ugh.
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      08-20-2021, 08:28 PM   #6
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Quote:
Originally Posted by im_an_alien View Post
That is definitely the rear diff, and looks like the bushings are shot. PITA to replace..
I think your shop meant the front bushings on your differential
Thanks for confirming! All this at just 23K miles on the car .
Any idea if its covered under CPO warranty?
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      08-20-2021, 08:30 PM   #7
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Had no idea those bushings were fluid filled. Since OP is still under CPO best let the dealer handle it, I believe this would be covered...
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      08-20-2021, 08:33 PM   #8
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Quote:
Originally Posted by im_an_alien View Post
Had no idea those bushings were fluid filled. Since OP is still under CPO best let the dealer handle it, I believe this would be covered...
…German Engineering🙄
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      08-21-2021, 05:19 AM   #9
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Those rear bushings are fine. The oil dripping from them is leaking out of the pinion seal of the diff. As the pinion spins, it flings the oil outward, hitting the exhaust, and the bushings. 100% a pinion seal leak on that unit.
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      08-21-2021, 06:29 AM   #10
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Quote:
Originally Posted by LA1Z24 View Post
Those rear bushings are fine. The oil dripping from them is leaking out of the pinion seal of the diff. As the pinion spins, it flings the oil outward, hitting the exhaust, and the bushings. 100% a pinion seal leak on that unit.
Wouldn't there be oil residue dripping down on to the diff housing itself? It's bone dry! Even the insert nut and flange doesn't have any residue...
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      08-21-2021, 09:21 AM   #11
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Quote:
Originally Posted by im_an_alien View Post
Wouldn't there be oil residue dripping down on to the diff housing itself? It's bone dry! Even the insert nut and flange doesn't have any residue...
It's being slung off before it gets there. It weeps past the seal, hangs out on the pinion flange (not really what it's called on a bmw) and as it rotates it gets slung outwards. Notice the oil is also on the exhaust at the same exact lateral location as the mounts and pinion.
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      08-21-2021, 01:05 PM   #12
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Quote:
Originally Posted by LA1Z24 View Post
Those rear bushings are fine. The oil dripping from them is leaking out of the pinion seal of the diff. As the pinion spins, it flings the oil outward, hitting the exhaust, and the bushings. 100% a pinion seal leak on that unit.
interesting observation, I think you're likely correct on this. It certainly makes not much sense that with such low mileage those bushings would be shot.
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      08-21-2021, 01:23 PM   #13
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Quote:
Originally Posted by LA1Z24 View Post
Quote:
Originally Posted by im_an_alien View Post
Wouldn't there be oil residue dripping down on to the diff housing itself? It's bone dry! Even the insert nut and flange doesn't have any residue...
It's being slung off before it gets there. It weeps past the seal, hangs out on the pinion flange (not really what it's called on a bmw) and as it rotates it gets slung outwards. Notice the oil is also on the exhaust at the same exact lateral location as the mounts and pinion.
Plus one on this
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      08-21-2021, 01:51 PM   #14
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Quote:
Originally Posted by LA1Z24 View Post
Those rear bushings are fine. The oil dripping from them is leaking out of the pinion seal of the diff. As the pinion spins, it flings the oil outward, hitting the exhaust, and the bushings. 100% a pinion seal leak on that unit.
Thanks, is pinion seal an extensive repair?
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      08-21-2021, 03:57 PM   #15
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Quote:
Originally Posted by Wgosma View Post
interesting observation, I think you're likely correct on this. It certainly makes not much sense that with such low mileage those bushings would be shot.
I've been a tech for over 20 years...Now I'm not saying it can't happen, but I have never seen a mount, for something like a diff, that is oil filled. Engine mounts yes, others, not so much.
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      08-23-2021, 08:16 PM   #16
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Quote:
Originally Posted by LA1Z24 View Post
Those rear bushings are fine. The oil dripping from them is leaking out of the pinion seal of the diff. As the pinion spins, it flings the oil outward, hitting the exhaust, and the bushings. 100% a pinion seal leak on that unit.
Dealer service also diagnosed it to the pinion seal of the diff. Replaced under CPO warranty, no hassles.
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      08-23-2021, 08:41 PM   #17
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Glad it's sorted. Now that I think about it, even if it was the differential mounts, what are the odds of both failing at the same damn time? That's what happens when you rush to conclusions....
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