1. #1
    bradleyheathhays's Avatar
    bradleyheathhays is offline Senior Member
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    300k mile old diff fluid

    I have a '11 Crown Vic with 300k miles on it and my rear diff fluid looks like this...

    300k mile old diff fluid-shfyg7b.png

    Been watching some utube vids on how to change this fluid out but after a visit to the local Advance store I've got some questions. Up til now, forums on the net have led me to believe that all former police cars (which is what mine is) need 75w-140 fluid with friction modifier, but after giving them my VIN at Advance the computer comes back with a fluid suggestion of 80w-90.

    Here's the diff tag...

    300k mile old diff fluid-17kjzfr.jpg


    I believe that shows a 3 (L or 1)? 27 which indicates a 3.27 gear ratio, and then an 88 after, which indicates the 8.8 axle. Not sure what that SYN means, unless it means synchronous?

    This car is driven VERY sparingly, long distances at 60-65mph but never hard

    , and operates in a climate that can get up to 95F in the summer.

    When the rear is jacked up and one rear wheel is spun, the other moves in the opposite direction.

    So my first question is... what weight fluid should I use? Also, I need confirmation if I should use a friction modifier.

    Next is how to go about changing the oil out. I'm being lazy here and after watching a vid on how you can extract 80% of the diff fluid out the fill hole by using a flexible hose I'm thinking about just doing it that way. But if you guys think it's necessary to get all that old fluid out I'll take plate off and do it the regular way.

    And lastly, if I take the plate off, should I spray all the gears down with brake cleaner to get all the old fluid out, or just let it drain for a while and call it good?
    Last edited by bradleyheathhays; 04-30-2019 at 09:33 PM.

  2. #2
    JPNinPA's Avatar
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    300k mile old diff fluid

    You could syphon most out. If it looks nasty. You will want it all out.
    But it is best to crack it open and wipe out all oil and metal shavings.
    If you crack it open just let it drain. Do not use cleaner...


    Being the other tire spins opposite its unlikely limited slippery differential.
    Oil weight for that rear should be:
    80w90

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    Last edited by JPNinPA; 05-01-2019 at 06:24 AM.
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  3. #3
    TerryMason's Avatar
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    Re: 300k mile old diff fluid

    I've got a ford 8.8 in the back of my Wrangler. The friction modifier is only if you have a limited slip, and as JPNinPA mentioned, it doesn't sound like that's the case.

  4. #4
    bradleyheathhays's Avatar
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    Re: 300k mile old diff fluid

    Thanks for the good info yall. A few follow up questions...

    I trust ya JPN but why no cleaner? I've had others advise to spray it all out, I really don't know one way or the other.

    Also got advice that I might have a limited slip diff but the clutches are worn out. I suppose I could write ford to get a build sheet for it.

    What advantage is there to using the more expensive Mobil 1 synthetic as opposed to a cheaper Valvoline full syn... $20 a bottle vs. $13, times 3.

    Also, I've been given advice to replace the clutches and 'steels?' in the diff as mine are probably worn out. What advantage in performance would I gain by doing this and is it expensive/ worth it?

  5. #5
    JPNinPA's Avatar
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    Re: 300k mile old diff fluid

    Once you open it you could see if you have limited slip.
    Chemicals can damage the seals. It will break down the new oil if you donít get all the cleaner out. Also, if you have clutches or new clutches the cleaners will damage them.
    And unless water got in there, letting it drain and wiping is sufficient. If water got in the oil is more like pudding.

    Donít really know on the brand though people like Royal Purple and Amsoil

    Never had to replace them in my vehicles.

    Limited slip delivers torque both tires when one is slipping. Dry roads better launching.





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  6. #6
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    Re: 300k mile old diff fluid

    I replaced the clutch pack in my 8.8's limited slip. It was pretty easy my first time, but it does take a while. You'll have to pull the diff cover, rear brakes, pull out the axle shafts, then take the carrier out. Inside the carrier you'll add new metal plates that stick together when pressed. It can all be done with common hand tools. The result is kind of like a locker, but much weaker.

    I honestly don't think you'll need a limited slip on a car with 300k miles, unless you're trying to race people at stop lights.

    You can test your limited slip by jacking up one rear wheel while the other is on the ground. If you can easily spin the tire in the air, then your limited slip is worn out.

  7. #7
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    Re: 300k mile old diff fluid

    If you are changing the oil and have the pumpkin open, check and see if it has this S spring - this is what applies pressure to the clutch packs

    300k mile old diff fluid-15847d1357679549-terrymason-s-2005-jeep-tj-build-12-stock-ford-8.8-carrier-terrymason-jeep-wrang.jpg

    You can kind of see the clutch packs in the right of this photo

    300k mile old diff fluid-15848d1357679549-terrymason-s-2005-jeep-tj-build-13-stock-ford-8.8-lsd-terrymason-jeep-wrangler-.jpg

  8. #8
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    Re: 300k mile old diff fluid

    If it looks like Terry's good. Just wipe it out.

    The below needs some cleaner.



    Regards,
    JPNinPA

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  9. #9
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    Re: 300k mile old diff fluid

    I agree .
    Needs to go yonder.

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