Ruined rear differential…how do I prevent this from happening again?

Title. To elaborate more, I had my 2005 mustang gt inspected by a mechanic shortly after buying. He found that one thing that needed fixing was a small leak from the pinion seal in the differential. I ended up buying a duralast pinion seal and royal purple gear oil and had him do the job. Simple enough. A week after the repair goes by with moderate driving and the leak seems to have stopped. No more spots on the driveway like before. All of a sudden, yesterday, my car starts making a horrible grinding noise. Like a metal on metal scraping sound. There were no noises whatsoever before yesterday, it came on extremely sudden. I take it back to the mechanic and we find the brand new pinion seal is leaking and the differential is almost empty. He tells me I need a new differential now.

My question is: how do I prevent this from happening again? Obviously I will never buy aftermarket seals ever again, (I know I’m an idiot for that) but other than that, in terms of symptoms, there were no obvious warning signs until it was too late. The car went from running perfectly fine one day with no noticeable leaks or noises to the next day making a horrible scraping noise and by the time that occurred and I took it to the mechanic the differential was ruined. I also hadn’t seen any spots in the driveway that would have indicated a leak so what I’m guessing happened is that the seal started leaking very rapidly over the course of one day (yesterday) probably WHILE DRIVING, and not while parked, hence me not noticing any oil spots in my driveway to warn me the seal was leaking. Again, the car wasn’t making any noise before the grinding suddenly started. Not even a whine. I specifically remember yesterday in the afternoon I was running errands with the windows rolled down and no noises at all. Then later that night when I was driving home on the highway from my friends house it was making the scraping/grinding noise. It did not come on gradually. So essentially there was no indication the differential was almost dry before it was too late.

Did I just get extremely unlucky? Or am I missing something.

The mechanic overtightened the pinion nut when the seal was replaced which caused the pinion bearings to fail. The seal was not at fault.

You need to rebuild the rear, not replace it.

Or, rather, your mechanic needs to rebuild it but I doubt he has the skills.


Like you mentioned, I’d avoid (I do avoid) anything Duralast. Could be the seal wasn’t installed properly, though. Seems odd that the pinion seal would let almost all of the gear oil sling out just by leaking. I mean I could see it if the seal wasn’t in there at all, but…

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Can overtightening the pinion bearing cause the seal to leak? Because when we looked under the car that was the only place it was leaking from. Also he told me rebuilding it would be more expensive than replacing it with a used one

That seemed very odd to me too. You would think a. leaking seal would be pretty gradual. I just don’t know how I can prove that this happened because of a mechanical error on his part. (ie overtightening the pinion bearing) He told me it’s likely because I brought him an aftermarket part.

I do know he uses some alldata program or something like that that tells the exact steps for a repair including torque specifications for bolts down to a tee and follows everything by the books so I highly doubt he overtightened it

The only time I’ve seen that occur is when the PBP wasn’t done correctly.


Probably not, But overtightening it did cause the bearings to fail, and that’s what caused the seal to leak.

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So you all are saying the only way this could have happened is from an error on the mechanics part? (Overtightening the pinion bearing) And not the seal just being bad? Because I want to be completely sure before I go to him making an accusation like this. He’s been doing this longer than I’ve been alive and has done nothing but good work for me for the past 5 years

Is the fact that the differential went from full to almost empty in 2 weeks a sign that there was a big error made? Or is it possible for a bad seal to leak that rapidly?

Could be that the fluid level got low enough to damage the bearings before the new seal was installed. Could be that bad bearings let the pinion shaft wobble around enough to ruin the old seal, then the new seal quickly failed for the same reason. I’d think the mechanic would have, or should have, been able to detect any damage already done before he replaced the seal, though. Seems pretty coincidental that it started getting all noisy shortly after he did the work, though. I don’t like Duralast parts from past experience, but they probably have decent enough pinion seals. Not a real complex part. In all honesty, I don’t think the fact that you used an aftermarket part caused your problems. But I don’t think I would say for certain the mechanic messed up. I’m not an expert, though. Just a shade tree mechanic at best.

In my case the bearings failed causing the pinon seal to leak.

Would you say the most common cause of a pinion bearing failure is overtightening? Or is this something that can fail with age as well?

This is a tricky scenario. It sounds like a bad pinion bearing can definitely cause the pinion seal to go bad. But also a bad seal can cause a leak and thereby cause the bearing to go bad due to low fluid. So it’s a bit of a circular issue and very difficult to prove which one was responsible for the other failing. Obviously if the bearing went bad (which more than likely would have been the mechanics error) then I am entitled to request the repair be done free of charge. I just don’t know how I’d prove this

I guess the main way to tell is: would a failed seal due to a bad pinion bearing cause a much faster leak from the seal as opposed to a pinion bearing that’s torqued and functioning properly but with just the seal that’s leaking? Since I seem to have lost all the fluid in just a day or so

Ask the mechanic what they set the PBP at.

Then watch the expression on their face.



Was it leaking before the rear started making noise? Or after? As the pinion bearings start to fail, the seal will leak since the pinion shaft moves around.

You posted “new” not used. Yes a used rear would be cheaper but shops tend to replace all the seals in the axle before they send it so you risk the exact same problem occuring again if the mechanic makes the same mistake.

I’ve done this repair. If you do it by the book, it takes quite a while. Mechanics rarely do it the way Ford says to do it. There is a faster way that works fine BUT if you over drive the pinion nut with the impact wrench (that everyone uses) the bearings will fail.

It had to have been leaking before the noise, although there weren’t any spots under my car when I parked it overnight. So it had to have leaked out while driving.

You replaced the pinion seal because it was leaking. Perhaps the oil level was low enough at one point that the bearings and gears in the diff already had some wear that wasn’t obvious when driving.

Or perhaps there was already some “slop” or play in the pinion bearings and that’s why the seal was leaking in the first place. Then, retightening the pinion solved the issue temporarily until the bearings finally gave way.

Diff failure can be caused by over or undertightening the pinion. Or also can be caused by 15 years of “spirited” driving, as Mustangs are prone to. If this is the rear end I think it is, it’s not particularly stout.

A used replacement would definitely be cheaper, but you’re also getting something that’s as used as yours was before it failed.


Even when it was leaking originally before the first repair the diff never made noise so it’s reasonable to conclude that there wasn’t any major damage. You mention diff failure can be from over or under tightening the bearing. While it is possible the bearing just happened to give out from age, I find it very coincidental that it gave out a couple weeks after having it worked on. Would you say it’s reasonable to assume it was overtightened?

For the 3rd time… YES it is reasonable to assume it was over tightened. Everything you describe screams that loudly.

Nope, I wouldn’t assume anything because I didn’t inspect the car before or after the work was done. At this point I don’t think you can prove anything other than your car needs a major repair.

I’m really not trying to point or not point fingers here, but the truth is there’s not enough info to come to any conclusion, other than the car had a part failure shortly after it was worked on. Note I say “worked on” and not repaired. You bought a part and asked a mechanic to install it. Not trying to be evasive here, but even a simple job like replacing a pinion seal isn’t as simple as it seems. There’s a process and a system of “best practices” to be followed, and it seems though it wasn’t.

At the shop where I work, the mechanic would have:

Taken the car for a test drive before any work is done, to listen and feel for any noise/vibration/harshness and noted anything on the pre-repair inspection, with pics if needed. Inspected and verified that the seal was leaking.

Removed the rear diff cover and drained the fluid, inspecting for any shavings or discoloration in the fluid. Also would have inspected all the gear surfaces for wear. Perhaps measured backlash if he felt it was needed.

Only then would he replace the pinion seal. Following that he would have taken the car for another test drive and rechecked for noise or leaks. If at any point the mechanic noted anything unusual, work would have stopped until the customer was notified.

It sounds to me at this point that you have no way of proving the diff wasn’t making any noise or had any wear, and the mechanic has no way of proving that is was noisy or worn. It’s simply your word against his.


It’s a GT and entirely possible that it was a bit low on gear oil and shockingly; may have been whaled on by the prior owner(s). Hard to believe someone would thrash a GT…

I have a Jekyll and Hyde personality so I get it. In my Lincoln or Sonoma I’m a docile driver. In a turbo SAAB with a 5 speed manual the gloves come off a bit… :slight_smile:

I know it doesn’t help now… but I would have had the car inspected by my mechanic BEFORE buying it. It sounds like this was a pre-existing problem that may have changed your mind on buying the car.

Be that as it may… I’d take the car to another mechanic and have it fixed. Let Mechanic #2 diagnose it, and also order the parts to fix it.

Good luck.