Leaky rear differential

[M3 Driver]

i am accumulating oil on the deck below my rear differential. i have not investigated whether it is an axle, drive, or cover. does the e46 M3 have a predictable failure pattern here?

[COVID-19]

They usually leak from the axle seals and input shaft.

[M3 Driver]

should i replace all three seals? i will be taking it to a service facility.

[Johnvu]

Yeah just replace all three while you have it out. They make kits to replace the clutches as well but that will likely cost a fair amoun more. Good luck with everything!

[M3 Driver]

i got a quote from my local independent BMW shop for all 3 seals and he wanted $950 total with $575 for labor and including $234 for a new rear diff cover. the remainder is the seals and fluid. he said the aluminum warps with heat. the labor seems really high and i am not inclined to replace the cover. what do you think?

[Cubieman]

Get under the car and isolate the problem, if its simply the cover take a whack at it yourself, that job is not too bad. The other things require the diff. comming out and apart, thats a good job the shop if you don't feel comfortable doing diff. seals and have the necessary tools to set backlash etc. You could remove the diff and reinstall to save a bit of money.

[M3 Driver]

does the differential have to be pulled?

[Cubieman]

I suppose not if just doing the seals.

[Shadytype]

I did the input seal while it was still on the car. Thats where mine leaked. Not sure about reqr seals or the cover. Assuemdly tje cover is a no, because it has two integrated mounting points

[ATB88]

Can do the seals with diff in place. A little annoying to get the axles out of the diff, and the new output seals into place, with the limited space. But you'll get it if you're patient and figure out which directions to push/pull.

Front seal requires taking some underpanels off, dropping the exhaust section 2, getting the exhaust heatshielding off, and dropping the back half of the driveshaft, and undoing in the input/pinion nut before getting to the seal itself. Not hard but not a trivial number of things. For the output seals I think you just need to unbolt the rear swaybar mounts and rotate it out of the way? Also probably need the section 2 out of the way for the driver's side seal. But again it's tight/annoying getting the axles out of the diff output flanges. May require some tapping with a mallet.

If you do it yourself, I can not emphasize this enough when doing the front pinion seal: MARK *EXACTLY* WHERE THE PINION NUT SITS ON THE BOLT BEFORE UNDOING IT AND TAKING IT OFF TO GET TO THE SEAL. WHEN UNDOING THE PINION NUT, COUNT THE EXACT NUMBER OF ROTATIONS IT MAKES BEFORE IT COMES OFF. IT DOES NOT HAVE A TORQUE SPEC AND IT NEEDS TO GO BACK ON EXACTLY THE SAME WAY YOU FOUND IT OR YOU CAN DAMAGE YOUR DIFF.

[Cubieman]

I did the input seal while it was still on the car. Thats where mine leaked. Not sure about reqr seals or the cover. Assuemdly tje cover is a no, because it has two integrated mounting points

I hope to do the diff. cover with the car in the air and something like a floor jack supporting the diff., wasn't planning on removing the whole thing just for a cover.

[eacmen]

does the differential have to be pulled?

As said before if you're just doing seals then no. But depending on your garage (lift vs stands) might be easier to do it correctly by dropping the diff. Getting access to the input shaft you're already doing like 90 percent of the work to drop the whole diff. Might as well refresh the front mount bushing and do the seals at the same time.


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[M3 Driver]

i need to establish where my leak is originating from.

[eacmen]

i need to establish where my LEAK is originating from.

How much oil we talking about? Diff capacity is only 1.2 qts. Do not run the car if you think the diff may have run dry.

Back the car up onto some ramps. Should be obvious where the leak is.


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[ATB88]

i need to establish where my leak is originating from.

You said that oil's accumulating on the "deck" below the diff. By that, it sounds like you mean the part of the rear subframe that passes through beneath the input of the diff where the diff meets the driveshaft (which is also the piece where you would jack up the rear of the car from). If oil is accumulating there, it sounds like the like is coming from your input/pinion seal. Leaked fluid at the output shafts wouldn't make it to the "deck" you're talking about (if we're talking about the same thing). A leaky input seal is relatively common on these diffs after a long enough time. That seal one of the only real problems my car has had. Do check your output flanges for leaks too, though, as those tend to leak eventually as well.