I've had my rear differential fluid replaced twice over the last 4 years or so (the car is almost 7 years old) and both times there have been large metal chunks stuck to the magnetic drain plug. Magnetic drain plug from rear differential with pieces of metal attached

At this point, it's pretty clear that the rear diff is going to fail at some point.

How will I know when the rear differential fails and what are the potential consequences if I let it fail before replacing it?

  • Teeth tend to break when subjected to shock loads - such as drpping the clutch at 4k rpm to do wheelspins or burnouts.... – Solar Mike Feb 2 at 8:24
  • @SolarMike hmm, I'm not launching the car. And so far, it's only the rear diff. Everything else (front, center, trans) is fine as far as I know. – Ellesedil Feb 2 at 8:34
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    Even loosing then regaining traction can cause a shock load sufficient to break teeth... – Solar Mike Feb 2 at 8:38
  • @SolarMike that definitely could be plausible. – Ellesedil Feb 2 at 8:39
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    Your image is quite telling. When you stated you had "chunks", that was an understatement to me. With what you have showing, I'd suggest you are looking at complete replacement at this point and utter destruction when it does let go. There shouldn't be anything which looks like that coming out of the diff at fluid change. – Pᴀᴜʟsᴛᴇʀ2 Feb 2 at 12:27

When the rear diff fails, one of two things can occur which will let you know it's gone south:

  1. Noise: - When driving, you'll be hearing a lot of noise from the rear, especially when going around a corner. It could start out by it having a lot of whining, with it continuing to get louder over time. This usually happens when the bearings start going bad, which makes noise. If the bearings get bad enough, they can allow the alignment of the gears to go bad, which will make even more noise. If it gets beyond that, see #2 below. This is the regular way a differential would fail (for the most part). Losing all or most of your gear oil exacerbates the whole situation.
  2. Total Carnage: - The other way you are going to know is if it fails spectacularly. What I mean by that is, the differential itself completely comes apart inside (or possibly outside, too) of the differential housing. There will be a lot of chunking and possibly it will completely lock up. It might blow up (using the term "blow up" generically, meaning, it stops working due to broken parts) and allow the tail shaft to spin, but the car goes nowhere. Either way, the differential is through. This is less likely and usually happens when a lot of horsepower is put through the differential, or when wheel hop occurs at the racetrack.

Differentials should last the life of a vehicle under normal conditions. As stated in #1, losing gear oil helps this along significantly, so if you have a blown pinion seal, get it replaced. This is not as straightforward as it might seem. The pinion has a preload, which if not done correctly, can cause gear misalignment, which causes a lot of noise and excess wear.

In your case, having chunks on the magnet is not a good thing. Getting it fixed sooner rather than later is a good thing as the longer you wait, the more damage will have occurred. The least you should do (IMHO) is to get it inspected so you can know for sure what's going on. The only way to do that is by getting the rear cover off. You won't know until you inspect it.

If you let it go to failure, you are going to be costing yourself more money. Depending on what's wrong, you might be able to get away with getting the bearings replaced. If you wait until failure, you're more than likely plussing up the cost ten fold.

  • Hmm, my assumption/expectation was that the only real fix would be full replacement. The Subaru performance shop I frequent doesn't really bother with rebuilding rear diffs. The car is modified now, going from 265 stock hp to about 300ish/310ish if I had to estimate based on dyno readings. However, it was still stock power-wise when the diff fluid was first replaced and I haven't heard of any rear diffs failing in the Legacy community, which includes some cars pushing a lot more power than I am. I have no known leaks. – Ellesedil Feb 2 at 0:50
  • @Ellesedil - Like I said, you don't know the carnage until you take a look. Adding 35-45hp is not a huge leap. Depending on how you treat the car will give you an idea how long it will last. If it isn't making noise, you probably have some life left in it. If it's already whining, time is short. As far as rebuild versus replace, the advice I gave is generally what happens. Subaru diffs may be cheaper/easier to just replace, I couldn't tell you. – Pᴀᴜʟsᴛᴇʀ2 Feb 2 at 0:56
  • I had two front differential failures in a 2000 Legacy Outback wagon, bone stock, so I won't assume they will last the lifetime of the vehicle anymore. Ended up replacing the entire differential housing the second time and so far it's been okay. My son is still driving it at almost 270,000 miles. I suspect there was a manufacturing defect in the housing. It was quiet until explosive failure, two teeth stripped off of the ring gear each time. – Dacid Salin Feb 2 at 5:56
  • @DacidSalin - In my answer I stated it "should" last the lifetime of the vehicle. It's not an assumption, but rather a statement of what is considered normal. There are always exceptions/outliers. Sounds like your's held together quite well since! :o) – Pᴀᴜʟsᴛᴇʀ2 Feb 2 at 12:24

I think from the junk you show that it is the limited slip that is the problem . Hard to guess what may happen ;Worst is that the junk gets between the ring and pinion and " total carnage". I agree with @Paulster , the basic ring and pinion last "forever" without the junk, although they may "whine" due to wear or minor misalignment.

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