Strange rear end leak

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Aug 23, 2004
Well, I now have a strange rear end leak. A month ago when I dropped the transmission, as I pulled the driveline, I got a few tablespoons of gear oil drip out of the front of the yoke. I also noticed the yoke bolts were damp. The main seal is dry.

It's not leaking from the outer seal. The only thing I can compare it to is like when you install a harmonic balancer and don't use sealer on the keyway slot, and you get an internal leak from the bolt rather than the outer seal.

Well, it looks like it has gotten worse and oil has slung onto the exhaust pipe and under the truck. (Gotta love that gear oil smell)

I removed the driveline again this morning, and more fluid spilled out. I stopped by oreillys and picked up some rear diff permatex, and smeared some in between the flange and flange nut and then worked an o-ring in there. Pressed in there pretty good and let it cure for a coupe of hours. I'll see if that stops the leak.

Anyone ever had a similar leak?? Does the diff need to be looked at by a pro?

What seals the innards form leaking?
plugged vent? pinion seal?

I went through 2 pinion seals in 75000 miles on my 2002 4.0 4x4 ranger

although when they went they mostly just seeped and dripped off the diff.
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By "main seal" or "inner seal" you must be talking about the pinion seal. The pinion seal is the only oil seal in the pinion shaft/gear housing of a rear end, and it is merely pressed into the mouth of the pinion shaft/gear housing. A pinion seal is composed of both metal and rubber. The perimeter edge of the pinion seal which is pressed into and contacts the mouth of the pinion shaft housing is metal with a rubber seal adhered around the perimeter edge. The pinion seal also has a mouth, composed of rubber. The pinion flange or yoke (some rear ends have a flange and some have a yoke) has a sleeve that contacts and rides along the mouth of the pinion seal.

Here is a picture of a pinion flange and pinion seal: (the grey plastic thing around the perimeter of the pinion flange sleeve is the dust cap and is pressed in.)

Here is how they go together:

This is the first time I have ever heard of a pinion oil leak through the splines of a pinion yoke/pinion shaft and around the pinion nut. These things are on tight! And require a puller to remove. The reason there is no oil seal required in the first place is because the splines on the pinion yoke and that on the pinion shaft mate together so tight that it is not required. Are you sure this is not just a simple pinion seal leak? If it did not leak before why would it leak now? As there is no movement between the splines of a pinion yoke/pinion shaft and there should be no wear?

Either way, if you remove that pinion nut make sure you mark both the nut and pinion shaft together with a punch or permanent marker. If you decide to remove the pinion yoke you will also have to count and measure the exposed threads on the pinion shaft, that yoke must go back into exactly the same position as before or you will ruin your backlash setting.
Like I said its a weird one. So the only thing sealing the front yoke to the pinion is a tight fit?




Here it is after I worked on it a bit, but you can see the main seal is dry, and the oil is either coming from the hole near the u-joint or simply between the two mating flanges.

I would say the diff was about a cup low.
Originally Posted By: JTK
Could the vent be plugged and it's building pressure, forcing gear oil out?

Not sure, I went out to take a look at it. It is just a hose that plugs into one of the axle tubes and runs to the frame up high with the other end open. no dirt or anything up there.
You're right that is bizarre, and it doesn't appear to be leaking from the pinion seal. The only thing I can think of is the pinion nut may have loosened off due to vibration, and the pinion flange with it. If the pinion flange is loose riding on the pinion shaft that may be the source of that strange oil leak.

Here is a link to a DIY pinion seal replacement on an F150, you can see for yourself how everything fits together:
For GM's they say to use RTV on the yolk splines but I haven't done that to my yota and silverado but yours is a special case so I might try RTV'ing the splines.
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