Leaking Rear Diff- 08 F250

akf250 · Nov 28, 2012

Noticed a loud humming sound from the rear of the truck when I drove it after it sat for a week and I'm pretty sure my diff case is dry (no fluid on the ground but the bottom of the diff is evidently covered in old oil) Is there any recommendations on new seals or gasket sealer that should be used when replacing? Also what oil would you recommend and how much? Would a aftermarket Mag-Hytec seal that much better?

6.0 p-stroke · Nov 28, 2012

A leaking pinion seal is pretty common, I can't think of the proper fluid to use when you replace it but I'm pretty sure it needs to have some sort of friction modifier in it. You probably don't need to replace the diff cover, but if you pull it off make sure you clean the surface and have some good RTV to use to reseal it.

086.4 · Nov 28, 2012

I havnt changed the fluid in my SD yet but all my other trucks i used mobil one synthetic gear oil and i have always used ford motocraft friction modifier it stinks but works the best and its not all that expensive.

Zmann · Nov 28, 2012

the spec is on a tag and it needs to by SYN

i would stick to the OEM or Mobil 1 but keep the recommended viscosity and use the friction modifier as needed to cut chatter if u have a limited slip ..

but if u ran it dry ,, u got problems IMHO

and yes the modifier stinks like heck

TrueBlissCustoms · Nov 28, 2012

We're running royal purple in our truck.

91turbogsx · Nov 28, 2012

75w-140 Synthetic for the rear diff. I see alot of synthetic gear oils that don't require a friction modifier to be added.

And that Ford friction modifier stinks my whole shop up for a week after I use it...:doh:

duddy · Nov 28, 2012

Royal purple 75w-140 with the limited slip modifiers FTW

akf250 · Nov 29, 2012

Thanks guys, picking up some royal purple and parts tomorrow. Any advice needed in draining and refilling the diff besides making sure the gasket seals properly? How much exact oil is needed?

Zmann · Nov 30, 2012

no gasket on the aluminum 08 cover if u have that and not sure about the steel version

I just use RTV

I can't remember how much oil it took ?( u can ask the parts place or look in your owners manualIIRC) but u fill to the upper fill plug until it seeps out

91turbogsx · Nov 30, 2012

Buy 4 qts. It's about 3 1/2 to fill it. And I always use RTV for the covers.

mandkole · Dec 1, 2012

how did it go? did the noise go away after the fluid change? Silicone on the cover and clean everything REAL GOOD with lacquer thinner before putting on the silicone, then let the silicone set overnight if you can before putting the fluid in. Never had a leaker

Zmann · Dec 2, 2012

I have yet to meet anyone who has the patience to let the sealant set up overnight LOL

I feel like I am going nuts waiting 10 minutes but I know most shops don't wait any time at all

duddy · Dec 2, 2012

I have never heard of waiting, brake clean it and silicone it. then start dumping in. Never had a leak myself. Except for when i forgot to silicone the bolt for the pinion adapter flange.

akf250 · Dec 2, 2012

Got the oil drained and replaced. Of course I didn't wait for the sealant to set overnight lol but no leaks so far. Problem NOW is that I still have this weird humming noise- everything inside the diff. looked great and I only had very small metal shavings on the drain plug so no problem in there.

It's sounds exactly like a truck with big offroad tires going down the road- a constant humming note that changes with speed- lower the speed more noticeable- higher the speed less noticeable as the humming pitch becomes constant. I can slightly feel it in my steering wheel so I'm beginning to think its happening on the front side of the truck. Everything through the drive line looks good, no rubbing, leaks, etc. Transmission is freshly built like 10,000 miles ago and is running great, so I don't think its internal. Maybe a hub's going out? Any other bearing or things you could think I should check?

Maybe my brain cus I'm going crazy lol- I noticed it right when I got back in and was like to my buddies- "You guys can here that. Right?" and of course no one else can tell a difference so it makes me look crazy lol UGH the fun of diagnosing a weird sound...

duddy · Dec 2, 2012

I bet the radio can fix it! :swordfight:

Zmann · Dec 2, 2012

don't worry what ever it is will get to the point you will know exactly what it is LOL

superstroked · Dec 2, 2012

Axle bearing?

Zmann · Dec 2, 2012

ince He know thinks it may be upfron I was thinking unit bearing

I had a rear axle bearing go ,, and u could see it in the diff fluid even before it made any noise

91turbogsx · Dec 2, 2012

I've had a unit bearing go on the '08 and it hummed like you are describing. But i've also had bad pinion bearings on the same truck sound the same way. Just gotta pinpoint whether its from the front or rear.