Rear diff smear

[Jonny09]

Ok folks, riddle me this...
2009 JKU Sahara. Original rear diff and cover. Cover’s a fair bit rusty and been sprayed with undercoating by prior owner.

After going to Battlegrounds two weeks ago I noticed a wide stripe of oil/fluid running up the cover over the rear diff. There was no oil on the garage floor, but running my finger along the bottom edge of the diff cover some relatively clean oil came off on my finger (almost like it was waiting to drip). Ok, I thought, it got hit A LOT on the ride and I figured I messed with the gasket seal.

I didn’t drive for a week but regularly crawled under and checked. Still no drips on the garage, smear of oil still on the cover, and just a touch of fresh oil clinging to the bottom. Ok, same diagnosis, but not leaking enough to make me afraid. I’ll get to it. Week 2, same results, but drove the Jeep a bit.

Before I fixed it, I went to Brookridge yesterday. Coming home I started getting anxious that I hadn’t replaced the gasket yet and promised myself I’d do it today. Today I crawl under the Jeep and the bottom of the diff is bone dry. There is still a smear of oil up the back of the cover mixed with grime and rust. I clean off the cover as best I can. Chip off rust (soaked in oil). Scrub the whole thing down thinking maybe it’s cracked or lost integrity. Nope seemed solid. poop, was it leaking more than I thought and it’s dry? I remove the fill plug expecting to have to use a dipstick to find the level of my leaking fluid but instead diff fluid comes steadily dribbling out the fill hole. WTF? Fluid is full. Additionally, the diff fluid seemed a darker color than what I was getting on my finger.

Question of the day: Where did this oil smear and drip come from on the cover of my rear diff? With all that was mixed in the rust and grime there was a fair amount. I can’t figure it out!
Any thoughts?

[Axeman]

Does it look like the cover peeled back at all at the bottom from sliding over a rock? Or is there fluid coming down from the Pinion seal?

[MDSRACING398]

If it is really rusty it can be seeping straight through the metal. I have a couple decent 44 covers if you need one.


Or just like getting into a Helicopter in the military if it isn't leaking don't get in it's out of fluid.

[Jonny09]

So I drove to Home Depot this evening (about 5 miles total). When I got home I checked the bottom of the diff. Had a slight amount of oil on my finger. Gotta believe I just smacked the diff cover @ Battlegrounds and tweaked the gasket. Don’t know why it was dry after yesterday’s run but I’ll pull the cover off tomorrow, clean it up, reseal it, and see how it goes.

[Frank]

Id also check and see that the vent hose/nipple didt give you just a burp. Sometimes when the diff is hot and you go into cold water crossing. It does weird things. See if the end of the vent tube is wet. You should be able see if the cover peeled back on a scrape. Most of the time the factory seals the cover on so the gasket per sey is not gonna move , neither is the gasket. Start looking for an aftermarket cover. Stock cover are good enough for most speed bumps in a parking lot only. Another person suggested its coming from where the drive shaft hooks to the diff. Crawl forward a bit and check there also. FjR68

[ChrisL]

If it is really rusty it can be seeping straight through the metal.

I feel that Mark is on to something, Post a photo of your cover, it is wet in a strip over where the ring gear goes. It is very strange, I don't know how seepage from the bottom can make that strip.

[Jonny09]

So I finally took the afternoon off and played with the diff.
Here's a pick of the before shot:
This image looks much worse that it was originally because this was after I wiped it all down and cleaned it off last Sunday. I just smeared the oil around more as I wiped it off. I was convinced that although the integrity if the metal might be bad that the leak was probably in the bottom. Wind from under the vehicle would catch the drips and splat them up the back of the cover (like running in the rain gets the backs of your legs covered in road crap). I was further convinced of this because yesterday I noticed some minute oil spots on the underside of the muffler.
I took the cover off and this is what I found on the inside:

You can see how filthy it was. The fluid came out a nasty DARK grey/black. I guess it may never have been changed in 106000 miles. (To protect myself I state I bought it at 103000.)
Anyway, I wiped it out and apparently did not take a pic. It was pristine and cleaned up 100%. There was one tiny blemish about 1/4 inch long up near the top but I suspected it was just a casting blemish. The entire inside looked like new.
I went ahead and ground down the outside taking off the rubberized rust-coating and rust. I thought it looked pretty good. I didn't like the pockets of rust you can see in the next pic but again, thought it seemed structurally sound, at least enough to treat the rust, paint, and put back on until i decide to regear or install lockers.

I brought it inside and gave it a good wash before I settled in to paint it. As it was drying from the wash I was bothered by that one blemish on the inside. It appeared to line up directly with the deepest of the rust pits. As I ran my fingernail over it I felt a slight roughness so I decided to take a chance. I grabbed an old pointy knife we keep around the kitchen and very gently touched the blemish. There was definitely a groove. pushing a little harder the knife slide easily into the blemish. Rotating the knife, with just the tip in the groove it produced a small hole that was clearly a weak spot, and likely a sight of seepage. I though all along this might be the case, but really, really, didn't think so. I was wrong!


Mark wins the prize!
Now I have a new cover on order and Jonathan is out of commission for a week while I await delivery.
On the other hand, any day you get to play with the Jeep, learn something, and make progress is a good day!

[RandyCarol]

That’s all it takes. If you ordered a stock cover grind down the bottom edge so it doesn’t extend past the axle housing, this will prevent peeling back the cover when dragging over the rocks. That’s another common cause of leaks

[Jonny09]

That’s all it takes. If you ordered a stock cover grind down the bottom edge so it doesn’t extend past the axle housing, this will prevent peeling back the cover when dragging over the rocks. That’s another common cause of leaks

Awesome suggestion! Thanks Randy, I can do that. God knows I've dragged the rear diff across enough rocks!

[pirahnah3]

Well glad it was a simple fix for you, kinda stinks that it means your down for a bit thou. Might I ask why you didnt opt to order some stronger aftermarket covers?

[ChrisL]

Now I have a new cover on order and Jonathan is out of commission for a week while I await delivery.
On the other hand, any day you get to play with the Jeep, learn something, and make progress is a good day!

If you want to come to Westford, I can give you a free stock cover.

Chris