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Water in the rear differential

I have a Ford F150 3.5 ecoboost XL FX4 and am wondering if I need to worry about the barings and the seals. Took in my truck for front, rear and transfer case fluid change and they found a small amount of water in the fluid (rear). The mechanic said the seals allow water to get in but does not allow fluid to get out. No fluid has leaked out and no damage inside the differential. No play in the axle.
A mechanic friend of mine says water got in the seals and barring need to be replaced or water could have got into the vent.

What do you experts think?

I would have got water into the differential likely from driving through a low water crossing (creek crossing).

Check that the differential breather hose is still attached. Item1.



Oh its definitely possible to get water IN to the diffs… no doubt about that especially if you know that you have been doing some water crossings.

Yes it can and does get inside of the diff…and yes it does NOT like to leave… it gets incorporated into the gear lube and turns it into a milkshake color.

All you need to do is change the diff lube…say 2 times… that should carry all the moisture laden lube out and replace it with fresh lube. You dont need to be overly worried about damage since this is early detection and honestly it still has all of its gear oil present as well. Could it damage your bearings? Sure maybe…over time…and if you left it the way it was…and if it sat for long periods of time unused…but its not something you should fret over.

Just change the lube more than one time… and avoid adding more water to the diff (with your river crossings) and you will be fine I’m sure.

If you cannot avoid the water crossings…you may be able to put a ball valve or some other device on the vent for the diff…shutting it off just for the water x-ing…then returning it back to normal. Hell its nothing a pair of long nose vice grip pliers cant do to squeeze the vent tube shut temporarily, or simply route the tube to a higher elevation…but why simplify things when you can have so much more fun by over complicating them?


This is one of those cases where it would have been better to post pictures of the fluid

What one guy says is a bunch of water . . . another guy might say that is normal condensation because the diff fluid is still stone cold due to the truck sitting all night. Drain the fluid stone cold. Drain the fluid hot. It won’t look the same. Not at all

Fortunately I got a sample of the rear. I am sending it out to Blackstone.


Have you made any effort to eliminate the contaminated oil? If not you might consider doing so soon. By the time you get a chemical analysis of the oil determining how much water is in it a lot of damage can occur to the bearings which will eventually trash the axle shafts, ring and pinion gears and the spiders, i.e. everything other than the housings and cover.I would recommend that you immediately remove the cover and empty the housing then replace the cover and fill with light motor oil and with the rear axle elevated run the engine with the transmission in low gear for a few minutes, then open the cover, drain and refill with the proper gear oil…

Every bearing in the axle is heavily loaded regularly when driving and any corrosion will become a big problem soon or sooner.


And how did you get that sample?

Did you siphon it out?

Did you remove the diff cover?

Did you get the sample cold?

Did you get the sample hot, after driving the truck for a few miles?

And can you post a picture?

In all honesty, I suspect you don’t have a real problem here

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I’m not an expert mechanic, but I’ll offer my off-road and water crossing advice (from past experience):

  1. Don’t cross water crossings higher than your axles.

  2. If you disregard #1, change the differential fluids at your earliest convenience.

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Yeah I was going to suggest the same thing that Mr @Rod_Knox did… with draining the fluid and adding an “intermediary” fluid like a light motor oil to basically serve as a diff flush. Then just refill with the proper lube. Its a good idea.

What is not a good idea is waiting for some fancy analysis… what do you expect them to tell you? Its not like an engine oil analysis that can tell a myriad of things… You have water in the diff… get it out, right away, sooner the better.

In my prior post I mentioned the worst thing that you could do is to leave it in there and also not drive it…that gives it time to separate and or dry the bearing surfaces out etc… Just drain the fluid… immediately.


I’d do that, except use the same fluid throughout.

The sample was taken with the cover removed. Siphoning does not remove enough. I should have taken a photo, but didn’t. The mechanic said it had very little water, he has seen where they are a lot worse, he didn’t see any differential damage beyond normal wear. The fluid was replaced with redline fluid (mfg specs 75W85 per manual). He just wanted to notify me that it was found.

If after driving several miles you pull the fill plug and stick your finger in and find that the oil is the correct viscosity and not a muddy paste you might have dodged the bullet.


Great idea! :+1:. He mentioned doing that with my transfer case as the fluid was fairly burnt. Didn’t think about doing that for the diff.

Guess I am lucky. I drove through water higher than my axles many times due to flooding and low grade boat launches, before getting differential fluid on front and rear, plus transfer case fluid at 165k on my trailblazer, no note of water from my bud that is a gm mechanic who does work for me on the side.

It all depends on the condition of the seals and the vent arrangement. I once dunked my TB up to the tailgate opening launching and retrieving at a particularly difficult launch due to water height issues and the ramp design. The vent was most certainly submerged for 10-15 minutes combined time. The axles are regularly submerged when launching which is sometimes several times a week. After the last event, I drained it and was surprised to see zero water and pristine diff lube. The vent tube design looks good for what small amount of air exchange happens but little incentive for water to go down it. I suspect if I left it that way for much longer, some would eventually go in as it slowly bubbled out air…the axle and pinion seals appear to be in good shape as well…

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That’s what I was thinking of recommending.

Living in Florida, I’ve considered getting a vehicle with decent ground clearance, like a Jeep Wrangler, and outfitting it so it can be driven on flooded roads. I’d install a snorkel on the air intake and a tube on the rear differential’s vent and run it to an elevated point near the back of the vehicle.

What else would you recommend the OP and I modify to protect such a vehicle from water penetration? (I ask in this thread, rather than starting a new one because such modifications might be a good idea for the OP after finding oil in the rear differential.)

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According to my Jeep friends @Whitey they all say to simply stay out of the water if you can because Jeeps aren’t submarines. However if you MUST go thru deep water, they simply suggest you raise the vent tube to a higher elevation AND…after known encounters fording deep water that you go and check your fluids for the presence of water contamination. They are not shy about dropping the fluid in each diff…xfer case and transmission…not shy at all about it.

So I guess the real solution to this issue is to avoid it if you can… and be vigilant if you cannot. Thats about my assessment of that situation.


Right, and that’s good advice, but I’ve found myself driving through tropical storms and tropical depressions in my Honda Civic. The trick of driving through standing water of any depth is to avoid it if you can, and drive as slow as possible, barely creeping along, if you have to in order to not create a wake that can splash water into something it can damage.

With all that in mind, and with the problem of living in Florida among rising sea levels, I’m considering planning for the future to have a vehicle that can survive the inevitable flooding, and safely evacuate from a flooded neighborhood.


If the vent tube extends several inches upward and then makes a loop downward submerging it will not result in water pouring in. Of course if that tube is split or is loose at the axle housing fitting it will leak…

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Yeah, that’s why I mentioned the arrangement being important. On the TB, it basically just goes up “high” on the frame and is affixed there. The candy cane approach wasn’t used. Even so, air has to come out for water to go in and the tube size and breather cap help to prevent that from happening over the short term…