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Filling Rear Differential - Should I Add More Fluid? #5381697 03/20/20 10:02 PM
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Fitz98 Offline OP
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Limited Slip 373 9.75 Ford Differential.
I put in 3 quarts M1 75W-140 LS friction modified gear oil with (1) 4 oz bottle of Ford's friction modifier.
I probably wasted 8-10 ounces with pre lubing gears, axle bearings, axle shafts and splines before and during reassembly, with probably 2-6 ounces wasted from overflow, spillage, etc. while lubing parts.
When filling, with the vehicle level, the fluid did not come out of the filler hole. My understanding was that this is the stopping point when the system is full.
I cannot see how far below the filler hole the fluid is, and cannot tell by feel with my finger.
My options:
1) Leave as is, knowing there may be a few ounces less than 3 quarts of fluid, plus the friction modifyer
2) Buy another bottle of Gear Oil and fill until it comes out of the filler hole as originally targeted
3) Add another 4 ounce bottle of Ford Friction Modifier (which I have), keeping in mind I have already added 1 bottle to the already fortified gear oil I have used.

I want to do it right and price is not an issue.
What would you do?

Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381699 03/20/20 10:12 PM
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Leo99 Offline
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I'd buy another bottle of gear oil and fill it until it drip out. You came this far. Might as well finish the job properly. Of course, you'll probably put in a tiny amount and it will start to overflow. But at least you'll know it's done right. Using a pump is much easier if you're not already using one.


Without data you're just another person with an opinion. W. E. Deming

2003 Corolla 250,000 miles (RIP)
2004 Corolla 125,000 miles
2004 Rav4 392,000 miles
2015 Camry 90,000 miles
Re: Filling Rear Differential - Should I Add More Fluid? [Re: Leo99] #5381701 03/20/20 10:14 PM
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4WD Online Confused
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Get another quart and top up to the proper level.

Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381703 03/20/20 10:23 PM
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slug_bug Offline
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Yep, Option #2


2017 RX350 (42k)
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Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381704 03/20/20 10:24 PM
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Blkstanger Offline
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If you can't feel it with your pinky finger you need more.


99 Jeep WJ 4.7 200,000mi. Maxlife 10w30, Fram Ultra.
15 Ford Escape S 2.5, 75,000mi. Castrol Edge EP 5w30 Fram Ultra.
Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381713 03/20/20 10:54 PM
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PowerSurge Offline
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You definitely need more, like the others said. Fill it until it is oozing out of the fill hole.


1993 Porsche 964, 2001 Ford SVT F-150 Lightning, 2004 Chevrolet Corvette, 2018 Toyota Tundra and a few others
Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381753 03/21/20 01:00 AM
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john_pifer Offline
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Does it specifically call for filling until it starts overflowing?

I know my Tacoma calls for the level to be a little bit below the fill hole.


07 Tacoma V6 - M1 EP 5W-30, Fram Ultra, 10K OCIs, 225K
18 Civic HB 1.5T 6MT (Fiancee's) - VME 0W-20, OEM filter
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Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381791 03/21/20 05:11 AM
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demarpaint Offline
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Fill it to the proper level. You want it level with the lowest point of the fill plug, [ starting to running out] or a hair below which you would easily see or feel.


God Bless Our Troops

Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381835 03/21/20 07:35 AM
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atikovi Online Content
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Find the PROPER specs. Some cars it's to the bottom of the fill hole, some it's 1/4 to 1/2 inch below. Too much is as bad as too little.

Re: Filling Rear Differential - Should I Add More Fluid? [Re: john_pifer] #5381865 03/21/20 08:03 AM
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OP, why were you pre lubing?

Originally Posted by john_pifer
Does it specifically call for filling until it starts overflowing?

I know my Tacoma calls for the level to be a little bit below the fill hole.


That sounds crazy to me. Why didn’t they just design the full hole a little lower?!


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Castrol Magnatec 5W-30 D1G2 + FU filter (2nd OFI) (77,760mi, 3/7/20)
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Re: Filling Rear Differential - Should I Add More Fluid? [Re: gathermewool] #5381868 03/21/20 08:05 AM
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atikovi Online Content
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Originally Posted by gathermewool
OP, why were you pre lubing?

Originally Posted by john_pifer
Does it specifically call for filling until it starts overflowing?

I know my Tacoma calls for the level to be a little bit below the fill hole.


That sounds crazy to me. Why didn’t they just design the full hole a little lower?!


So you don't make a mess on the shop floor.

Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381891 03/21/20 08:28 AM
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Gebo Offline
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Option #2

One thing that I learned is to give the fluid time to settle down before checking.

I noticed when this when I filled my diffs until they dripped out and immediately reinstalled the plugs.
I went back to check them the next day and none dripped out. Apparently, it takes a little
while for the fluid to level out. YMMV


'98 LEX LS400 305K
'02 4Runner 250K
'05 Lex LS430 90K
'07 Lex GX470 70K

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Re: Filling Rear Differential - Should I Add More Fluid? [Re: atikovi] #5381950 03/21/20 09:33 AM
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gathermewool Offline
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Originally Posted by atikovi
Originally Posted by gathermewool
OP, why were you pre lubing?

Originally Posted by john_pifer
Does it specifically call for filling until it starts overflowing?

I know my Tacoma calls for the level to be a little bit below the fill hole.


That sounds crazy to me. Why didn’t they just design the full hole a little lower?!


So you don't make a mess on the shop floor.


No, it’s a serious question? You don’t just drain and refill the diff?

Besides, it sounds like the OP spilled some anyway.


14 Forester XT Touring FA20DIT (Cobb Stage 1)
Castrol Magnatec 5W-30 D1G2 + FU filter (2nd OFI) (77,760mi, 3/7/20)
15 Legacy Premium FB25
Magnatec 0W-20 D1G2+ FU filter (2nd OFI) (54,621mi, 3/4/20)
Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5381965 03/21/20 09:50 AM
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Kira Online Content
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My vehicle calls for the "1/4inch to 1/2inch" below the fill hole.

It seems to me (and everyone else) that filling to the base of the fill hole couldn't make a difference.

Perhaps the gear oil churns too much at that level?

After seeing those "clear diff cover" videos it's hard to imagine it could make a dime's worth of difference.

Re: Filling Rear Differential - Should I Add More Fluid? [Re: Fitz98] #5382028 03/21/20 11:01 AM
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GoldDot40 Offline
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This is where I use a small Allen wrench as a level gauge. Insert the short end into the inspection hole and point it straight down...while keeping the long end somewhat level on the bottom edge of the fill/inspection hole. Pull the Allen wrench out to see where the level is.


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