[MrSeabass]

Very long story short - 150K miles 35i, been getting a gradual whirring sound while decelerating but more recently started hearing what sounds like a locomotive choo-choo while accelerating. No sounds when in neutral, all are speed dependent. Been working my way from the rear to the front of the car checking for bearing/CV failures and found nothing. Feared it was the front differential as I've heard diffs make that sound when dying.

So I replaced the fluid in the diff and, well...



Yep, those are little pieces of the differential unit. It sounds better with new fluid but it's clearly the overall culprit.

So asking if anyone here has replaced the front diff themselves as I'd like to go this route instead of taking it to a specialist. I know that no matter what this won't be a cheap fix, but I want to save wherever I can. Or if there exists any sort of rebuild kit for this. Thanks!

[bmwPower99]

So you have some metal particles, nothing huge. It's not like they have clutch packs in them, it's just massive gears. The front diff is very close to the engine so the fluid goes bad very quickly and probably evaporates off and gets low, it should be changed regularly. The rear diff on mine looked like new after 6 years, front was like yours. Possibly do another fluid change in a week or so to try and flush all the particles out. I wouldn't replace it until it gives you reason to.

[MrSeabass]

I mean it's still driveable as in the drivetrain is just fine, but the droning sounds - especially decelerating - are significantly loud, to the point where it sounds like an old manual car going in reverse. It absolutely will need to be replaced eventually, it just comes down to where and when, and I'd love to be able to do the swap myself.

Edit: when I have time I'll try to get a recording of the sound, but for reference, it sounds like the pinion preload is loose, very similar to this video:

The other sound on accelerating is closer to a warped brake disk (I can confirm this is *not* the sound, but it is very close to the same rhythm/pattern).

[bmwPower99]

Wow, ok that is bad. I could only find a f30 xdrive front diff DIY

[MrSeabass]

Thanks for the video; that's about as close as it'll get to seeing how I can do it. Other places list a removal guide as 'remove both CV joints/propshaft and then remove diff' but that's clearly lacking quite a few steps :/

[MrSeabass]

OK here's a recording of it all. I am on a gentle throttle to 0:25, then coast to 0:30, then throttle at 0:34, then coasting/braking after. Ignore the CEL as it's the evap/gas cap warning. You can hear the 'chugging' start around 15mph.

https://streamable.com/biryf0

[bmwPower99]

Adjusting the pinion lash steps below, You don't have to replace it. Look at the crap that came out the diff and they still repaired it.

https://www.mye28.com/viewtopic.php?f=3&t=142750

[383vett]

With 155k miles, I would find a lower mileage replacement and just bolt it in.

[MrSeabass]

Originally Posted by bmwPower99

Adjusting the pinion lash steps below, You don't have to replace it. Look at the crap that came out the diff and they still repaired it.

https://www.mye28.com/viewtopic.php?f=3&t=142750

Yeah that screams 'take it to a specialist' :/

Originally Posted by 383vett

With 155k miles, I would find a lower mileage replacement and just bolt it in.

This is likely going to be what will happen since new diffs are $2-3K.

[MrSeabass]

Minor update - every shop in like a 20 mile radius is booked out for a month :/ I don't have the time to wait that long, so I will probably attempt to remove the unit myself in my garage this weekend. Worst case scenario, if I can't get to the point of removal, I can at least document it and put it all back together.

[MrSeabass]

Well I gambled and lost Wanted to make one more trip to the auto parts store before attempting automotive surgery tomorrow morning. Right turn and - PING!

RIP differential. Time to see who has the part in stock that won't take a month to ship...

[383vett]

Wow, looks broken indeed.

[ThomasR]

As an owner of a 150k mile F25 35i myself I hate that you had this happen. Over the life of yours, how many times did you change the diff fluid?

[MrSeabass]

Twice before, neither showed any signs of wear or breakdown.

Also tried getting it off this morning but there's *one* bolt that I cannot easily get to either with a wrench attachment or an extension. The socket fit on it, but nothing else.
Even zip-tying those tubes out of the way wasn't enough (they were not flexible at all and both ends buried in the engine bay) And in either case, I had zero leverage, so I'll have to tow it to a shop this week :/

[ThomasR]

Don't give up on it just yet! Try one of these:
https://www.harborfreight.com/hand-t...het-67994.html



Super thin, flexible and 14+ inches long.

[ThomasR]

This one was blown also:

[MrSeabass]

Originally Posted by ThomasR

Don't give up on it just yet! Try one of these:
https://www.harborfreight.com/hand-t...het-67994.html



Super thin, flexible and 14+ inches long.

...that actually might do it. I'll try to grab one tomorrow, but I'll still entertain offers from around the area to take it in. Eyeballing it, I might still need to unmount/lift the motor, which is not something I wanna attempt.

[383vett]

The head of a 3/8 breaker bar is smaller than a ratchet and also has an angle. You might use one to break the bolt loose.

[MrSeabass]

I gave up, I would have to take apart so much other stuff that it's not worth it. The tools suggested above are out-of-stock, and beyond that I don't have enough leverage/swing under the car. I need the car running ASAP and I'm already gonna pay out the ass for a new unit anyway, so I'm gonna go full sunk-cost and start calling around again for a shop.

[HPR]

Originally Posted by MrSeabass

I gave up, I would have to take apart so much other stuff that it's not worth it. The tools suggested above are out-of-stock, and beyond that I don't have enough leverage/swing under the car. I need the car running ASAP and I'm already gonna pay out the ass for a new unit anyway, so I'm gonna go full sunk-cost and start calling around again for a shop.

Good luck. Even though I do most maintenance items myself and use an independent repair shop for things I can't, I've found that BMW means Bring My Wallet.

[MrSeabass]

Differential unit arrived yesterday, specialist installed it this morning and I drove it away this afternoon - a whopping $3400 lighter in the pocket.

At least the car drives like new again.

Edit: Gonna take it apart later to survey the carnage on the inside (BMW waived off wanting the core)

[MrSeabass]

Still taking it apart, but my theory of the pinion bearing coming loose looks correct.