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Technical Topics Mechanical Maintenance and TSBs: Break-in / Oil & Fluids / Servicing / TSBs and Service Bulletin Rear Diff Leak (2020 M340i xDrive)

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      12-09-2019, 09:13 AM   #1
BMW_Midlo_Man
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Exclamation Rear Diff Leak (2020 M340i xDrive)

So let's start this by saying - My M340i has been AMAZING to own thus far. It's gorgeous. It's beyond-my-expectations powerful. The speed and power has easily been underrated by my estimations. For me, it's the perfect blend of automotive prowess, driving dynamics, and modern technology & features.

But here comes my first mechanical gremlin... I noticed multiple drops of oil (equating to ~2 tablespoons of somewhat blackened oil in my garage. No smell to it, doesn't smell burned. But being that I only park this car in one specific orientation in my garage, I know the oil is coming from the rear axle area of the car along the centerline, and the most-likely candidate would be my rear-diff seal(s).

Car is only 7 months into ownership, with ~6,000 miles on it.

Cleaned up the drips, I've kept checking after parking multiple times and I have not been able to confirm whether any new drops have formed.

I'll be notifying BMW and taking it in for service, but I'm a bit uncertain as to whether or not they'll end up flashing the ECU (as per the open emissions recall) during my service visit, and I am still running the pre-July ECU (that, from what I gather has left me with a lot of unexpected power).
  • Is anyone else seeing rear-diff leaks on the new M340i?
  • Anyone have any other thoughts as to what it could be if not a rear-diff seal leak?
  • Has anyone been forced to have the ECU flashed per the recall while having other service work performed? If so, how has it affected your driving experience?
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      12-10-2019, 02:14 PM   #2
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I think you're stuck taking it in but you could of course get under the car and inspect yourself to try to verify where it is coming from. I have a May build and I'm not reading into these unproven claims of power reduction so would rather not have a leaky diff than worry about the tune rumors. Probably they will want to update it if there are any changes or recalls due. If you're so inclined, you could get a baseline dyno and if you feel there is a power change, re-test it.
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      12-11-2019, 07:40 AM   #3
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I've honestly thought about it. (getting a baseline Dyno). Not sure what I would get out of that other than the confirmed knowledge that power may have been de-tuned, so I doubt I'd put the time and money into that. It's not like it will give me recourse with BMW. Won't change anything. They've still delivered a good car.

I'll probably just end up letting them do what they have to do, and hoping any power-reductions are just in my head. Appointment this afternoon with BMW to take a first look.
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      12-11-2019, 07:59 AM   #4
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Quote:
Originally Posted by BMW_Midlo_Man View Post
I've honestly thought about it. (getting a baseline Dyno). Not sure what I would get out of that other than the confirmed knowledge that power may have been de-tuned, so I doubt I'd put the time and money into that. It's not like it will give me recourse with BMW. Won't change anything. They've still delivered a good car.

I'll probably just end up letting them do what they have to do, and hoping any power-reductions are just in my head. Appointment this afternoon with BMW to take a first look.
One guy claim there was a "power-reduction" who coincidently sells tunes to regain the lost power, then when pressed, backpedaled and conflated his earlier claim with the first gen 340i with a MPPK kit..

There is no power-reduction with the latest iStep update, that's a complete nonsensical claim. A 50hp loss would be noticeable to even the most inept driver.

If there was a reduction in power after an update, the internet would been screaming their guts out about it and marching to BMW HQ pitchforks and torches.

Take you vehicle in and have it checked and yes, more than likely they will update the software, since the diff operates electronically and a replacement unit would have to programmed to the vehicle.
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      12-11-2019, 09:44 AM   #5
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I have one of the very early ones, I put my order in February 28 and I had a unit allocated on the first order for my dealer in a large city, I than got a "emission recall" shortly after and noticed a huge decrease in power, it actually made me quite upset but I'm over it now... I think this detune is a real thing.
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      12-11-2019, 10:43 AM   #6
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Quote:
Originally Posted by Weiner0123 View Post
I have one of the very early ones, I put my order in February 28 and I had a unit allocated on the first order for my dealer in a large city, I than got a "emission recall" shortly after and noticed a huge decrease in power, it actually made me quite upset but I'm over it now... I think this detune is a real thing.
Are you sure it's not all in your head after all the hoopla you heard here..

Do you have any meaningful way to confirm this supposed "power lost" ?

I swear, every time one of you guys buy into this power lost myth, I think of this video below


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      12-12-2019, 02:26 PM   #7
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UPDATE TO THE REAR DIFF LEAK: So I went to my service appointment at the dealer, who confirmed that it actually isn't the seals on the rear-diff, but the rear-diff casting itself that is slowly leaking. The dealership stated that they had never seen it, and a replacement is on order from Germany. They can't see any obvious visible material defect to the casting, but that it was definitely leaking from the body and that my warranty definitely covers the defect.

REGARDING THE RECALL WORK and REFLASH of the Car's Programming - they performed all outstanding recalls while I was in the shop, which including refreshing DME, Dual Storage, Backup Camera (software), etc. I did inquire as to potential performance impacts that would change the behavior - and while I didn't really expect a different response, they certainly advised that they hadn't heard anything of the sort, and were not in any way aware of any performance reductions that would stem from the software flash. The way they are itemized on the service record is in shop-shorthand though, so it's not completely obvious what all of the recalls were for exactly, but I'm sure I could dig it all up if I tried hard enough.

So all this to say 7-10 day worth of shipping for the parts and then I'll spend a day in a loaner while they fix her up. (There's another part, unrelated to the rear diff, which I'll post elsewhere). At least it's still drivable in the meantime, she mostly seems the same after the updated programming, but I'm not taking her to the edge with the rear-diff casting being in a leaky state right now.
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      12-29-2019, 02:03 PM   #8
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My case appears to be leaking also, 4500mi will monitor.
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      01-02-2020, 09:26 AM   #9
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I've noticed it seems to happen right after driving if I've been aggressive with running through the gears. Not launch control, but let's say you floor it from a stoplight in Sport + and pull up in the garage at home shortly after - several strong drops of rear-diff fluids show up.

I've been keeping a piece of cardboard under the parking spot so I can monitor. I can go days of 'normal' driving with nothing showing up under the car, but again - one stoplight's worth of having 'gotten on it' and yeah - there are those damn black drops.

Being replaced under warranty this Thursday by the Dealer - Replacement diff was air-freighted from Germany.
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      01-02-2020, 06:42 PM   #10
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I wonder if it's been overfilled, mine seems to come from the top and drip down the center of the case
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      01-08-2020, 08:57 PM   #11
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Update: took it in clearly explained the symptoms and how to repeat it, same as yours.

They disagreed and said its the fill plug, I went to my purchasing dealer 2hrs away and left it 2 days.

Picked it up tonight and it leaks exactly the same. The anger is real.
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      01-27-2020, 01:48 PM   #12
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Just giving the update, the rear-diff-cover leak was apparently the right fix for my case. Since they replaced the rear-diff cover, As of yet, no leakage has been noted on the garage floor, even after a couple of heavy-handed drive sessions.
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      01-27-2020, 02:42 PM   #13
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Quote:
Originally Posted by BMW_Midlo_Man View Post
Just giving the update, the rear-diff-cover leak was apparently the right fix for my case. Since they replaced the rear-diff cover, As of yet, no leakage has been noted on the garage floor, even after a couple of heavy-handed drive sessions.
they're doing my whole diff, dropping off soon :/
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      02-13-2020, 08:14 AM   #14
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Update, diff didn't resolve problem. A new car in inventory also exhibited the problem. BMW is reviewing and devising a solution.

TSB states if you are to bring your car in for this they will inspect but not replace any parts as the problem is under investigation.
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      02-28-2020, 01:24 PM   #15
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Had the same problem. Took it to the dealer and they replaced the rear differential. So far, no more spots on the garage floor.
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      02-29-2020, 08:38 AM   #16
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Quote:
Originally Posted by mlassota View Post
Had the same problem. Took it to the dealer and they replaced the rear differential. So far, no more spots on the garage floor.
It will return if you drive aggressively from a stop, redesigned cover and vent tube are being tested on mine now.
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      03-07-2020, 12:56 PM   #17
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This was resolved with a reworked differential cover, vent was drilled out, elbow attached and a rubber hose for expansion added. It will not leak now.
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