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, 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?

 

Bet that $&@!! smells wonderful! I'd flush with Kerosene

 

Not familair with the 11 crown vic but unless you've had the car since new you can only assume the rear axle is original. It probaly is but I wouldn't trust any of the computors in these chain stores. I know on my SD I went through this confusion and there was some coding which I forget now. . I would guess the SYN would be synthetic but I've attached a ford chart. I think it has answers to what you have found on your axle . https://www.fcsdchemicalsandlubrican...axlelubeUS.pdf

It indicates the higher priced oil you knew about. Though it may or may not be needed now , it's 100k stuff. If it's original, it did it's job well. I know the price of it floored me but I had no choice on the SD. I also got mixed advice on Motorcraft brand and adding the modifier over other brands , Valvoline, Mobile One etc, with the modifier in it. Simply went with the store bought basic brand name over running to a dealer. Still pricey, ran me $80.00 plus the cost of liquid gasket doing it myself .Saving on price was only on the cost of the additive which wasn't much. I'd just pull the cover and let it drain off real good. . I'd be careful about disturbing too much if everything in there looks halfway decent. But that's probably just me and the if 'it ain't broke don't fix it' thing I've learned over the years. It should be a LS and the additive is/was put in there for a purpose.

 

i take the advice of a parts store counter monkey and his computer, and throw it rite int he garbage can next to the computer.
put the proper gear lube in with the proper friction modifier.
your car has a 3.27 geared limited slip differential.
and as said above, pull the rear cover off and drain it good. i usually just let it all drain out unless i see something i don't like , then i will wash it out with diesel or kerosene sprayed in out of a weed sprayer. .

 

Thanks for the advice guys. 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?

 

mobile1 vs valvoline difference is in the name as far as i am concerned.
as for replacing the clutches in the diff on a car with 300k on it, well i would not unless the car was showroom new shape.

 

X2 on the above post.

. Unless there some other reason why the rear axle has your attention, "probably" worn out isn't a good enough reason to tear into it beyond the cover removal. .

 

Looking at this from a different point of view, with 300k on the clock, I wouldn't even take off the rear diff cover if there's no indications of leakage from the rear cover or the pinion seal. I'd suck out what I could and refill with appropriate oil / modifier.

I would never suggest this on a newer component that had tons of anticipated life left but the car is about all used up.

I certainly wouldn't start flushing the transmission or cooling system at this point either. Drain and refill.