All:
I have an '86 F-150 4X4 (302 EFI). Yesterday I had it out in the driveway and noticed a leak from the rear differential. This is the first time that I have seen this leak. I have heard no noises while driving it, noticed no driving issues with it when I had it out yesterday doing errands. I am no expert but from what I have read this seems like a pinion seal has given out. This is likely well beyond my skills so if I take it to a mechanic is this a major repair? Also, I am the 4th owner of this truck and I do not know if the oil in the differential has ever been replaced. Is replacing the differential oil now kind of like changing old transmission fluid - that you are likely going to move gunk inside and it will destroy the gears within? I know nothing about this so any insights are appreciated.


Leak is coming from the pinion seal?



It is oily in the middle where the differential is.

 

Dip your finger in the drip and sniff it. If it smells like sulphur ie rotten eggs it's likely coming from the rear diffy. If you have a manual trans it could be working it's way down the drive shaft from a leaking rear seal on the trans.

Typical change out for gear oil is every 60k miles. Your seal area looks pretty dry and I would expect to see a drip forming right at the seal. I'm wondering how you drive this truck? Daily driver or occasional dump run and hardware store run? Seals can dry out if sitting long times and then drip a bit when used after sitting long times. I would open up the fill plug and check the gear oil level. Should be full to the bottom of the fill plug. If I don't know when the gear oil was changed out then I would change it with new gear oil.

 

One caution when preparing to drain the oil:

First make sure the fill plug can be loosened.

This is one of those things I have NOT learned the hard way, but have heard from others. If you first drain the oil and then discover the fill plug is seized, you’re stranded. The truck would be undriveable (other than a VERY short distance) so you could not drive it to a shop for repair.

 

Hey, I'm that guy. But thanks for passing the tip along.

The OP's driveshaft looks dry from the center bearing, so I'm thinking he'll be dropping the U joint, and replacing the pinion seal.

 

If it even has a drain plug. If this is a f250/350 and he has a Sterling rear, the nuts on the pinion tend to work loose after many years of use. That can cause it to leak.

 

Define “major”. I think a lot depends on what tools you have available. For a shop, this would be fairly minor and hopefully not too expensive.

You’ll most likely want an impact gun to break the big nut loose. You’ll definitely need a bolt-grip puller to remove the yoke. You’ll also need a seal puller, an inexpensive tool to (wait for it) pull the seal.

You may also find a groove worn in the yoke, where the seal rides. This can be easily repaired with a Speedi-Sleeve.

To install the new seal, you’ll need a block of wood with a hole drilled in the middle to clear the protruding shaft.

None of this is fun, as you’ll be on your back the whole time. Make sure you’re not working on a sloped driveway. Once you undo the driveshaft, that’s one less thing keeping the truck from rolling. Don’t trust the parking brake by itself. Use chocks, too.

Does any of this sound daunting? We can walk you through the steps if willing to tackle it yourself. But if your time and capabilities are limited, you may prefer to hire a shop.

 

You can get an estimate from 2 or 3 shops. I'm thinking parts and labor should come in at about $250.00 +/- $50.00

 

On a side note, if you are interested in doing mechanic work this is probably a good time to enter the work force as an apprentice. I know that several of the shops in my area are unable to find people who want to do mechanic work.

 

1986 F-150 4X4. Isn't that an 8.8? From the first image posted, that doesn't look like a 9".

 

I was thinking the same thing not a 9"
I dont think is it a hard job other than getting the nut off and back on with the right pre-load when tight.
Should not need a tool to get the old seal out, big screw driver should work.
Dave ----

 

All:

Thank you for responses! I am fortunate enough to have an oil change pit in my detached garage on my property so I can go down in the pit and look at this a bit closer and will try to check the level as you suggest. Should the oil level be filled at the "top" once the screw is taken off or is there kind of mark on the screw that shows if the level is low? (I have no idea what I am looking for). Can you "overfill" the differential like you can do with engine oil or transmission fluid, and if so, is that bad?

I do have a small transmission fluid leak around the seal of the pan which I have not done anything about other than watch the level. The oil pan seal is also starting to go - but that is another issue - but this leak is concerning since there is no dip stick.
You all are way out of my league with your knowledge - I have no idea what you are talking about with regard to a 8.8" or a 9" differential. (I can take some more photos if that helps).

FYI: I have owned this truck since 2017 and have put just under 7,000 miles on it so I average about 1,000 per year - it is really a dream truck for me - and I do use it but this Virginia-made truck "sleeps" during our Pennsylvania winters - except for deer season.
No one in my family worked on cars so I had no teacher and am really only am confident in tackling little things that I know I can not mess up. I do read this forum every day and look at your responses as you help others - and I have bookmarked the Garagemahal website.

Thanks again for taking the time to respond. I appreciate it.

 

Here's my 8.8, I believe the fill plug is on the passenger side about 1/2 way up on the front side. Maybe drivers side.


Here's the front side...that could be the fill plug in that dark spot.


Here's where I'm greasing the center carrier bearing slip joint, I greased all U joints too as well as the front end. I use Lock N Lube grease fitting, it locks onto the zert fitting.


Here's a better shot of the rear,


To replace rear diff fluid, you make sure you can remove the fill plug, first. Then put your drain pan under the diff and remove those bolts around the cover. You can get a gasket from the auto parts store. You can also use oil resistant RTV, let it set up a bit before pushing the cover on and tightening the bolts.

 

On the driver's side above the axle height a bit you'll find a square headed plug. If it's rusty apply some penetrant and let it soak in. The proper level is even with the bottom of the plug hole. You can't really over fill it as gear oil will run out. However it's not uncommon that someone gives it a little extra squirt and then puts the plug in quickly before oil can run out. So take a drip catcher under there with you in case when you open it up. If the level is right there so you can stick your pinky in there and feel the gear oil. Or use a mirror and a flashlight. Truck level, gear oil even with the bottom of the fill hole...good.

 

To properly reset the pinion bearing torque one or both wheels have to be off the ground. So working in the pit may not be possible for this repair. When the pinion nut tightens you have to check the rolling torque. With used bearings around 8 inch pounds should be the maximum. If you tighten it too much the bearings will burn up and too loose will cause a leak. Get a service manual that covers what you have an read it closely and you will understand what needs to be done and which tools are needed.
good luck

 

If you read the manual, your inch pound measurement is with the axles out. Axles and brakes will throw that measurment off. I just tighten them down good and never had a problem. The crush sleeve is already crushed to the correct dimension, so just tighten the nut against it. Don't try to over tighten it and crush the sleeve more.

Another good way to do it, if the nut is not loose, take a center punch and make a tick mark on the end of the pinion shaft, and another one next to it on the nut. Then just line these tick marks back up when tightening. If your nut is loose, then you just have to tighten the nut up tight and let it go.