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  #1  
Old April 30th, 2019, 18:50
bradleyheathhays bradleyheathhays is offline
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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...





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...



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, 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?
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Old April 30th, 2019, 18:55
bradleyheathhays bradleyheathhays is offline
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Re: 300k mile old diff fluid

Sorry, that large pic makes things hard to read. Not sure how to change that.
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Old May 1st, 2019, 20:39
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Re: 300k mile old diff fluid

My Amsoil catalogue says there's only one weight to be used, 75W - 140. The two types they sell are standard diff or limited slip, which you have. It also says synthetic is required.

With that many miles, I'd remove the cover. I don't flush mine, just drain all the fluid and wipe everything out of the bottom of the housing.
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Old May 2nd, 2019, 11:29
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Re: 300k mile old diff fluid

If the diff hasn't been opened in 300K miles, then if there is a limited slip in then it's a fair bet that it has probably been completely worn out for a long time...

Have you tried having Ford run the VIN and get a build sheet? Probably be more accurate
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Old May 2nd, 2019, 20:03
bradleyheathhays bradleyheathhays is offline
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Re: 300k mile old diff fluid

Thanks for all the good advice guys. I'lll write ford and see if I can get a build sheet like you mentioned.


Couple wrap up questions...

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?
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Old May 3rd, 2019, 08:39
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Re: 300k mile old diff fluid

3L27...L is for limited slip. If the other tire turn opposite then the limited slip is wasted.

SYN is for synthetic fluid.

And yes, you should be adding friction modifier...but it sounds like it's too late to matter.
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Old May 3rd, 2019, 08:43
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Re: 300k mile old diff fluid

Quote:
Originally Posted by bradleyheathhays View Post
Thanks for all the good advice guys. I'lll write ford and see if I can get a build sheet like you mentioned.


Couple wrap up questions...

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?
I wouldn't waste money on the expensive fluid or friction modifier unless you rebuild the limited slip (frictions, steels, springs).

Advantages...both tires get traction. Big advantage when drag racing, driving in rain, sand, dirt roads, snow, etc. For "normal" day to day driving ...not really any advantage.
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