Differential issues

[Gle8]

First, let me admit that I'm an idiot when it comes to cars (or anything mechanical). So the questions I pose may seem fairly ignorant, as they are. However, I know enough to know when I don't know what I'm talking about, so that's why I'm asking here.

Z4Mc (72k miles) is in the shop for some unrelated items. Shop says there is a major diff leak, and they need to pull and either rebuild or replace. My questions:

-With a leak, is this something to do with the housing or the internals?
-Is this something that can usually be rebuilt, or needs to be replaced?
-If I wait to perform the repair, can I cause further damage if things get worse?
-If I buy a new unit, do I need to buy the whole thing or just the internals/various parts?
-Anyone got a stock Diff for sale?


Thanks in advance for putting up with my stupid questions.

[The HACK]

Originally Posted by Gle8

-With a leak, is this something to do with the housing or the internals?

Housing.

Originally Posted by Gle8

-Is this something that can usually be rebuilt, or needs to be replaced?

Impossible to tell unless it's removed from the car, in my opinion. I mean, how major is the leak? Has the fluid pretty much ran dry for a while? Most of these will be answered only when the unit has been removed from the car and opened for inspection, IMO.

Originally Posted by Gle8

-If I wait to perform the repair, can I cause further damage if things get worse?

That's a resounding YES.

Originally Posted by Gle8

-If I buy a new unit, do I need to buy the whole thing or just the internals/various parts?

I don't think there's significant difference in price between just buying the internals and the whole unit. In fact, if you're buying used, i.e. from a scrap yard, or a rebuilt unit, they most often and likely come as an entire unit.

We'll probably need more information to give a solid opinion. Has the car been making some difficult to pin-point noises, especially when turning at speed? Sounds like knocking or humming coming from the back of the car that disappears when going straight? Where's the leak coming from? Has the leak been bad enough to leave a puddle under the car?

This might be a good opportunity to upgrade to a 3.91 ring and pinion set if the internal can be rebuilt. I'm just sayin'.

[Gle8]

Originally Posted by The HACK

We'll probably need more information to give a solid opinion. Has the car been making some difficult to pin-point noises, especially when turning at speed? Sounds like knocking or humming coming from the back of the car that disappears when going straight? Where's the leak coming from? Has the leak been bad enough to leave a puddle under the car?

I've only had the car a week, and I haven't noticed any odd noises coming from the rear at any speed, with the exception of some clunking from the driver's rear suspension when going over Philly bumps.

I also haven't noticed any puddles on the floor of the garage, which is why I'm surprised that the shop found what they describe as a 'really big leak'. I'm going to head over there and take a look myself before I authorize any work, which is why I wanted to check here first so I can at least sound informed...

And you're right, may as well do the upgrade now if they need to pull it.

[David70]

I really don't know but if it doesn't make any noise now, doesn't have any indications of any problems, what's the worst that can happen if the leak is fixed, it is refilled (might as well drain it completely first) and he keeps driving it?

[pokeybritches]

Depending on how "major" the leak is, you may be fine just filling it up every once in a while. I have a friend that was in your shoes after a PPI revealed a leak, but when we took the car to a specialty shop, we were told the leak was very minor. If anything, he should check the fluid levels every so often. He'll hit an inspection well before the fluid level gets low.

[GuidoK]

Originally Posted by pokeybritches

If anything, he should check the fluid levels every so often.

Originally Posted by Gle8

I'm an idiot when it comes to cars (or anything mechanical).

You think?

A diff should be dry. If it's wet on the underside, you should fix that right away.
There's only 1 litre of oil in there, and when it leaks, it's very hard to tell how fast it's leaking, or how hard it's going to leak.

There's not much to fix in the diff when it breaks unless used/aftermarket parts are sourced (bmw only sells complete units in casing apart from the seals). A remanufactured diff (from BMW) is about $3k, so well worth some care.

A major diff leak is probably the propshaft seal on a 210 diff (but it could be some other seal too, but seals are cheap, so you pay mostly for labour)
A good shop can inspect the friction plates and measure the backlash to make certain the diff is ok (not needed when there is sufficient oil left in the casing obviously).
Be sure to get a shop that has experience with diff overhaul: the drive flange nut must be marked before loosening (and afterwards screwed on in the same position).

[Finnegan]

Have a look at my thread. 10K miles later, no issues after it was fixed. Admittedly, it was fixed under warranty, but still.

If it's minimal like mine was, a new differential isn't required. However, finding a shop that knows how to repair (not just replace) a differential can be tricky.

Like The Hack said, might be a great time to install 3.91 or 4.1 gears.

Leaks aren't that uncommon. The question how big was the "really big leak". My skeptical side says to beware of comments like that from a shop unless you know them, or, know them by reputation.

[p1nk50ck]

Through some online research, I believe the previous owner of my car installed a 4.10 diff. I haven't had any luck in my attempts to try and reach him.

Is there any way that I can tell if the diff is still in the car or if it's been returned to stock?

I noticed that highway cruising RPMs are quite high in this car, but I've never driven a stock one, so I have nothing to compare to. Is there a noticeable difference in RPM levels from stock (like 65 mph in 6th gear should be XXXX rpm)?

[StickMon]

I'll try and remember to clock something on my way home tonight so you can compare.

[The HACK]

The RPM needle should point straight up at 80mph on the OE diff ratio. Not that I recommend you break the speed limit, just saying it's a good point of reference.

[StickMon]

Yep. 4th gear both needles straight up.
In a more conservative test, 3000 rpm in 3rd is 40 mph indicated.
My car doesn't do 65 mph in 6th.

[p1nk50ck]

Thanks. I'll have to check these out.

[p1nk50ck]

80 in 4th is 5500 rpm

80 in 6th is 3500 rpm