Hello all,

I am the proud new owner of a 2005 F150 FX4 with the limited slip rear end. The truck has 89k miles.

I recently changed rear differential oil and replaced it with Royal Purple 70w145 oil. I added one 4oz bottle of friction modifier I bought from the local Ford dealer. I understand the Royal Purple has friction modifier in it already, but I added the 4 oz bottle as well.

Now whenever I accelerate from a dead stop or accelerate during a turn, I feel a vibration in the rear end. I called the local Ford dealer and they said I needed to add 2 - 4 oz bottles of the friction modifier. I've driven probably 600 miles since I've changed the oil and added the 4 oz bottle of modifier. The mechanic said it's the clutches in the rear end binding...

This just sounds "fishy" to me. Any advice would be appreciated.

Thanks!

Brandon

 

Maybe it has something to do with adding an oil with a friction modifier plus the Ford stuff?? If I were you, I'd change it out again with whatever is recommended in the manual and see if that fixes it. Also, check the fluid level. Maybe you didn't add enough.

 

I changed mine about a month ago with Red Line 75W140 for LS and added 3/4 of a bottle of Red Line FM. Just did an over 2000 mile round trip this past week without any issues at all. How much lube did you add? Might have to play it safe and change it out again...make sure nothing got in the teeth while you were changing it and clean out the casing with a clean rag before you seal it up again.

 

After more research, I'm sure my clutch pack is going out of my truck. Same diagnosis: small shutter when starting out from a dead stop, will do the same when turning at low speeds.

I only added 1 - 4 oz bottle of friction modifier and the diff in my truck calls for 2 - 4 oz bottles. I am going to add another bottle, I'm curious to see if this will work....

 

I just put in RP 75W90 and also added 1 bottle of the ford friction modifier. 2000 miles no problems so far. Could it be that your slip yoke needs to be greased?

 

Check TSB 04-22-7
http://www.thedieselstop.com/faq/949...les/042207.pdf
Some 1999-2005 Super Duty F-Series may exhibit excessive drive-away shudder or vibration, under moderate to heavy acceleration from a stop. This may be due to driveline angle.

and/or

TSB 05-26-11
Some 4x4 2004-2005 F-150 (excluding Heritage) and 2006 Lincoln Mark LT vehicles build prior to 7/1/2005 may exhibit a rear driveshaft slip-bump concern on deceleration or acceleration from stops.


I had the latter issue, I replaced all of my u-joints and greased my slip yoke with marine grease, it has been smooth ever since.