I have a 03 F150 4X4 with 3.73 LS rearend. It has 75000 miles on the fluid and was wondering if it would hurt anything to add the additive on top of the existing fluid? The rearend has a little shutter around corners. Would it help of hurt matters?

 

With 75k on the fluid, you would be far better off to replace the fluid and add the modifier to fresh fluid.

 

I don't think it would hurt anything to add it. After all, the fluid replacement schedule allows much longer use of the fluid under normal driving conditions.

 

don't think it would hurt to add. i think the original fluid is scheduled for a 100,000 mile change out, but i believe it would be wise to just go ahead and change it out. can't hurt anything and will give a better peace of mind. that's what i would do.

 

When I changed mine out at 104k, it was REALLY NASTY looking. I'd change it.

 

Would it get rid of the shuttering around corners though?

 

Yep. Mine was shuddering, and it hasn't shuddered since. I used BG 75W140 synthetic with a bottle of their modifier. Some synthetics already have modifier, you have to see whether what you use already has it or not. I would not throw modifier in on top of fluid that has 75k on it.

 

When my 2000 F150Scab4x45.4 that I bought new had about 80k miles on it, the right rear axle seal started leaking. The traction lok had also started chattering around the corners. Since I had to drain the diff to replace the axle seal, I replaced the fluid with motorcraft synthetic and added a bottle of motorcraft modifier. The diff still chattered. I then found out about a TSB that said the fix was to replace the clutches and remove 0.010" in shims from the clutch pack to decrease the preload. The Ford parts guy told me to just add one more bottle of modifier which I did and it fixed the problem. The truck now has 150k miles on it and still works great. I sold it to a friend.

My recommendation still is to add one more bottle of modifier unless it has seen severe service. The cover will leak afterwards if you don't use the right technique with the silicon sealer gasket. I would look for a real gasket if I changed it.

 

If it doesn't chatter with one bottle, don't add another one.

I dunno about the 9.75, but Autozone carries a Fel-Pro gasket for the 8.8. 5 bucks.

 

When change the fluid and add the additive, do i need to use a little less fluid to compensate for the additive or just put it the recommended quantity of the fluid?

 

It should take the 4 oz bottle of FM and 3 quarts of gear oil if it is a 9.75 diff. Just keep filling it until the oil level is just below the fill hole.

 

Pump about half the first bottle in, then pour the FM into the bottle and continue.