intermittent rear diffential leak


Apr 13, 2005
Kinda confused about this leak. It seems to only leak from this point where the driveshaft meets the rear differential IF ive had the car on jack stands or a jack at a steep inclined angle. (Like this weekend i was doing a fuel filter change and had the rear end raised quite a bit and it started leaking).

But if the car is on a flat level surface, i dont ever notice the leak. No spots on the garage floor. Any clues to why it would be leaking so intermittently?

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I think it is the pinion seal leaking. If you change the car's ride height you can mess up the pinion angle which will lead to premature pinion seal failure.
With the car at a "steep angle" the fluid in the pumpkin "sloshes" forward and rests against the pinion seal. If the seal is on its way out it would likely be the time to do it.

During normal operation, the seal is only exposed to incidental splash and is somewhat protected by the slinger.

I'd suggest seal replacement and you might also want to check the oil level. It may not be weeping in normal attitude because the fluid level is low...
Suggest cleaning the area and confirming the leak location. If between the flange and the diff, then the pinion seal is leaking.

There is a Ford TSB regarding oil migration through the threads of the Pinion bolt. The fix is to install RTV silcone between the pinion nut, pinion flange, and pinion shaft. Remember anytime the pinion nut is removed, then the pinion bearing pre-load has to be re-established.

Note, the design of the diff causes the oil to be pumped against the front pinion bearing. In fact, if you look closely at the inside of the diff, there is a channel above and below the front pinion bearing to allowing the bearing to be bathed in an oil flow.

I agree with trinity_gt, the oil slinger will tend to direct the oil away from the pinion seal during operation. The fact it does not leak when setting level may indicate the fluid level has dropped below the level of the leak.

Remember that the proper fill level is 1/4" - 9/16" below the fill hole. It is not fill until it runs out.

My vote is the pinion seal. If it were thread migration, it would be a small slow leak all the time. Where as a sudden larger leak when the car is tilted forward would indicate the pinion seal is the source.

FWIIW; I had the same problem on my 2003 GT. I ended up installing a new pinion seal and RTV on the nut. It was a lot of work. Recommend doing your research before attempting yourself for the first time.
So you can get both ends of the car up with jacks and jackstands? How exactly do you do that? I find it to be unpossible since when I lift one end the other end gets too low to lift.

yea its not that hard. if you jack the back of the car up first, the part under the drivers door is going to go up not down, only the part infront of the front wheels will get lower.
lol its really not that hard, when i filled my differential i didnt have the 2 extra jack stands so i just had the back on jack stands, and i jacked the front up from the front of the door area, and put pieces of wood under the tires until the car was level
Well, found out why it was leaking. Took the car to the shop for some 4.10 gears and when they were installing them, there was no oil slinger. The previous shop forgot to install it, and thats why it was leaking. :notnice: