Does this need immediate attention or am I good for another 40k miles?



Right Side

Left Side

 

Check your vent and make sure it isn't plugged, check the fluid level and make sure it's at the correct level and not overfilled. Brake clean off and monitor it, that's what we'd do and tell the customer it's a rear diff sweat.

 

Where's the vent and how exactly do i check the fluid level in a sealed diff?

 

If you look on the driver side of the diff we're all the brake lines are bolted to the housing there's a rubber hose roughly in the center of that bracket that connects to a fitting that's threaded into the diff. On the other end of that hose is a plastic fitting that can get plugged with dirt and not allow the diff to breathe so it over pressurizes and and vents out wherever I can which can cause those sweat marks. I would disconnect the rubber hose from the metal fitting on the diff take a plastic tube from a WD-40 can and make sure the fitting in the diff is clear you should be able to stick that plastic tube in there don't drop it and make sure that is clear then you can blow through the rubber hose and make sure it's not plugged it should flow freely. Then to check the level on the front side of the diff on the driver's side just above the webbing there is a 3/8 ratchet square plug that you can remove to check the level if you remove the plug and it pours out fluid it's over filled. Otherwise sometimes you can get your finger in there and you want the level just below the housing on level ground or you can take a zip tie Bend the tip 3/4-1" and stick it in there just to see what the level is at.

Ignore the bad grammar it's text to speech, lol

 

A great answer.

 

Really, that oil could be from anything and might not even be from the differential. Perhaps something dripped down from above, maybe some stray assembly lubricant or something.

I agree with just checking the fluid level and cleaning it with a solvent. If it comes back, then you know that there is an active leak or drip.

If you haven't replaced the differential fluid yet, it might be worth draining and refilling and putting a new gasket on.

I've had a lot of success with the LubeLocker style reusable gaskets. Like any gasket surface, the key is to ensure the surface is 100% clean of any remaining old gasket and especially do not scratch it. Any scratches are a potential leak path. (If you do scratch it, it's not the end of the world, but you may have to use gasket maker underneath the reusable gasket, which then makes it non-reusable.)

 

Regarding the vent tube, if you find it was disconnected for any period of time, I highly recommend changing the differential fluid. Any water that gets in will react with the oil and turn it to sludge, which will quickly destroy the gears from the extra friction and heat.

If your differential fluid is cloudy, that is a very good sign of water contamination. (If it's really bad it will look like a milkshake.)

Random aside:
Not sure about the F-150 in particular, but a lot of automakers are known to use one-way breather valves on the axle that let air out but not in. This normally works fine, but if you immerse a warm axle during a water crossing, the cooling of the axle can cause a partial vacuum to develop inside. This partial vacuum is normally relieved by a bit of air getting sucked through the axle seals, but if the axle is underwater, it will suck water through and milkshake the differential. Hence why a lot of off-roaders swap to two-way vent valves with remote vent tubes.

 

Thanks guys. I'll be doing this in the next few days.