Rear Differential gasket and fluid - Ford Mustang Forum
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post #1 of 17 (permalink) Old 04-17-2018 Thread Starter
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Rear Differential gasket and fluid

I have a 2014 GT with track pack 3.73. I recently installed the Ford Performance Differential Cover. After going back and forth with the parts guy at Ford dealer he insisted that this rear end uses 75W-85 hypoid gear oil. He showed me a printout showing that a 2014 Mustang GT takes this fluid. So I swapped my diff cover and put 4 oz. friction modifier and filled the rest up with motorcraft 75W-85. Filled till the fill hole and let it drip out a bit till it was below the fill hole and cleaned it up and drove it. No issues a first, maybe 2 or 300 miles, and 4 quarter miles passes in I noticed fluid all over the rear end. I stuck my finger in and the oil was still at a good level, and looking further it seemed to be maybe leaking from the vent screw on top of axle maybe from being a little over filled. i cleaned everything up again and drove it maybe 50 miles at most. The whole rear end is wet again. So i just purchased some silicone to reseal the differential and went to a different Ford dealer now and they insist the 75W-85 oil is only for the Shelby and I should be using 75W-140. So after this super long story my questions are, Has anyone had any issues with the gasket leaking from a ford racing diff cover? Has anyone had issues with the vent screw leaking? And what weight diff oil is every one running in GT track pack rear ends?


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post #2 of 17 (permalink) Old 04-17-2018
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5pntlow_ny,

It's deja vu all over again, as Yogi used to say!

The bearing cap studs have to be sealed with RTV sealant or Teflon sealant. I found out the hard way: no drips initially but the bottom of the pumpkin eventually got damp and a drip formed at the low point. I thought it was over filled but the level was a smidge below the plug on the pumpkin (NOT the fill plug on the diff cover!).

I have the 'Leak Lock" gasket, too and it is good as gold.

Must do list:
* The diff cover drain plug needs thread sealant.
* The diff cover fill plug needs thread sealant.
* The bearing cap studs need thread sealant applied.
* The level is checked via the front plug hole (1/8" below the bottom of the hole). The plug has pipe threads and does not need sealant but you can put a little on (about 3 threads up from the inlet end of the plug).

That will get you set up nicely for a leak-free differential cover.

There is another issue with these axles and it is the vent. Ford has a couple options: the turkey baster works and the 'long hose' (which replaced the turkey baster) works. The stock vent will leak a little gear oil and this small leakage will wet the axle over time. (Really bugged me.) The turkey baster needs a zip-tie on the bottom to tighten the joint so it doesn't leak (this is what I have had to do with my baster).

Amsoil 75W-140 in my Mustang's differential.

OK, you know what to do, lol.

HTH,

Chris


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post #3 of 17 (permalink) Old 04-18-2018
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Pretty sure my manual states 75W-140 and my buddy that works at the dealer verified it. Mobil1 75W-140 and Motorcraft friction modifier is going in mine when I get it out for the season, and I'm installing the Ford Racing cover while I'm at it.

Jason

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post #4 of 17 (permalink) Old 04-19-2018
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for weight, go with whatever your manual says . . . I'm pretty sure earlier years said 75W-90 and later years said 75W-140 and that is the source of the confusion. My local drivetrain guy says 75W-90 is "better" but not clear why. Going with the manual is always a safe bet.


For brand/type, I like the higher end synthetics like royal purple or mobil 1. Pay attention to whether it already has the friction modifier in it; if not then you need to add it. Generally not a good idea to add more if it is already in there; except if it "needs it" and I think you can tell if it needs it by doing some tight figure 8's and see if you get any chatter or binding.

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post #5 of 17 (permalink) Old 04-19-2018
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One of the explanations I heard (I believe I saw this on the F-150 forums) is that you go with the thinner weight oil if MPGs are important to you, and the thicker stuff if you care more about component longevity.

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post #6 of 17 (permalink) Old 04-20-2018
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Are you talking about this cover?

https://www.americanmuscle.com/frpp-...iff-cover.html

or this one?

https://www.americanmuscle.com/frpp-88-diff-cover.html

If your car is indeed a track pack it would have come with the first cover stock. Anyways I'm betting that the leak may be coming from the axle vent tube. I had issues with mine with the stock fluid so what I did to fix it is upgrade to the finned diff cover and relocate the axle vent tube from the axle to the diff cover. If you upgraded to the second cover then you cannot relocate the axle vent tube. 75w-140 like everyone else says is whats recommended, although you can run thinner gear oil if you want. I am running amsoil 75w-110 in my car currently but am going to switch back to 75w-140 as I am supercharged and want to keep my stock diff alive for as long as possible. I didn't notice any mpg gain with running the thinner oil BTW. If the leak is not coming from the axle vent then do as the others have said and make sure everything is sealed up properly. When I swapped my diff cover I used a lube locker gasket from LMR instead of RTV and it hasn't leaked at all. Much easier than using RTV and waiting for it to dry.

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post #7 of 17 (permalink) Old 04-21-2018 Thread Starter
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nyuk98gt thank you for the detailed info lol. Now that you mentioned it it does look like the drain and fill on the cover are sweating a bit. I will definitely have to put some thread sealer on those. I will also do the bearing cap screws as well. I'll run the 75W-85 a bit longer seeing that I just spent almost $100 on it before i swap back to 75W-140. I don't drive the car too much and I maintain it often. This is probably the 4th time I've changed diff fluid and have 9400 miles. It just seemed weird to me at the dealer but the guy insisted that Ford made the change to 75W-85 for 14 GTs. My axle vent screw definitely looks wet around it, I noticed i could turn it either direction by hand very easily. I snugged it up, is this supposed to be tight or is it supposed to stay somewhat loose like that to actually function as a vent? I'll clean everything up this weekend and put some sealant on all threads and give it a test drive and let you guys know what happens thanks! and to fuelforfire87 its the 2nd one not the finned one

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post #8 of 17 (permalink) Old 04-22-2018
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5pntlow_ny,

Glad to help a fellow Mustanger out!

The axle vent cap spins around but there is a hex nut that holds the whole vent assembly into the axle tube. I don't remember the size but it is 5/8" or something like that. Bring a couple of wrenches under the car and check that it is snug. It doesn't need need to be really crunched down - the threads are pipe threads and they seal well once snugged up.

The fill plug is on the front of the pumpkin and a 3/8" square ratchet extension fits it just right. A little liquid Teflon sealant on those threads or Teflon tape or RTV sealant does the trick. Some don't like the tape because there is a risk of tape bits getting in the internal area. I don't know if that's a big risk or not but you can skip the first one or two threads if using tape.

Clean the drain area very well and then you can collect the almost new fluid and reuse if you want.. Look up the viscosity of 75W-140 and 75W-90 and see if there is much difference. If you don't run the car at the drag strip or race track then stick with what Ford recommends for the '14s.

I hope your sealing work bears fruit and there are no more damp plugs.

Don't forget to report back, please!

Chris
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post #9 of 17 (permalink) Old 04-24-2018
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Anyone aware of any good how-to videos to fully change the differential fluid out on these vehicles? It is something I would like to do eventually but all the talk through is hard for a visual learner like myself.
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Remove all the bolts on the rear diff cover,use a little screw driver to pry the cover away with a pan under it,clean off all the RTV sealent both on the cover and diff housing,I use a rag to wipe out the inside of diff housing, and to clean out the little gully at the bottom of diff housing.Next remove the fill plug on the right front of diff housing,you will need to get the little pump that fits onto the 75W140 bottles to make the refill easy ( 2.6 quarts.Buy 2 tubes of RTV sealant or buy the gasket to replace the cover,if you use RTV make sure you have a good bead of sealant on the diff housing and around where your bolt holes are,replace cover and tighten. PS I also like to REMOVE my axle vent and clean it in solvent, to make sure it is not plugged inside.Hope this helps you,keep us posted.
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post #11 of 17 (permalink) Old 04-26-2018 Thread Starter
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Lube locker gasket is on it's way as well as thread sealant. Will most likely pick up 75-140 from Ford when the parts arrive. I like to stick with OEM stuff. I work at a BMW dealer and in my experience using OEM parts and fluids and following recommendations given by the manufacturer is always the best way to go. I'll keep you guys posted once it's installed! And I also snugged up the vent screw, seems like no more fluid is coming from that area which is why I ordered the new gasket, its now obvious thats where it is coming from. I'm hitting the track May 7th so that should be a good test if it will leak again or not! Thanks again for everyones help and advice!
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Quote:
Originally Posted by JBert View Post
...Pay attention to whether it already has the friction modifier in it; if not then you need to add it. Generally not a good idea to add more if it is already in there; except if it "needs it" and I think you can tell if it needs it by doing some tight figure 8's and see if you get any chatter or binding.
It's always good to check even if it states that it already has the friction modifier in it. I used the Mobil1 that says the friction modifier is included on the bottle in a F150 with traction-lok that I had and it still chattered until I put the Motorcraft friction modifier in.

Jason

'14 F150 4x4 SuperCrew 302

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550 RWHP / 490 RWTQ
11.94 @ 123, 1.84 60 ft.
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I used Royal Purple,and was fine, didn't add anything and NO chatter.
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post #14 of 17 (permalink) Old 05-04-2018 Thread Starter
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I installed new gasket and fluid. Applied thread sealant to all drain and fill plugs and to the load bolts and nuts. I drove about 20 miles and smell differential fluid. From what I can see without jacking the car up yet it looks like it's coming out of the vent. Gonna have to figure something out for that vent.

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Quote:
Originally Posted by 5pntlow_ny View Post
. . . Gonna have to figure something out for that vent.
I put the Ford "turkey baster" vent thing in the vent hole up on top of the diff cover, and just let the axle tube vent clog itself. Or you could intentionally clog it with silicone or gasket sealer. I have had no more leaks with this setup, including road track time.


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