2004Lariat4wd

It looks like I've got a leak in the seal where my drive shaft meets my rear differential.

I took the cover off about a month ago, drained the fluid, sealed it with a gasket & RTV, and filled it with 75W-90 Royal Purple.

Is it possible to replace this seal without opening up the differential and having to replace the fluid?

djquik1

Yes, but you might want to put 75w140 in it since that is what it is spec'd for

2004Lariat4wd

In a previous thread (when I did the job), some members advised the 90 vs. 140 was not significant unless I drove in extremely hot temperatures and/or with heavy loads. At the time, I found conflicting guidance on what weight of gear oil was required when I compared Haynes mannual to service info provided by AAMCO.

Where can I get the "correct" answer?

Is the difference that significant?

djquik1

Factory Spec is 75w140 from Ford Factory manual, and it was on the little metal tag that was attached at one point to the rear axle.

The gear was designed for that weight of oil, and should be continued to be used. But your truck and Haynes is the expert.....rant over/

Sorry I work at a shop and deal with arm chair experts because of Hayne's manuals. We usually have to fix things after that because the customer can't get it right.

Can't tell you how many times we have to fix the timing chain tensioners and actuators because someone thought they could do it based on a "DIY" thread on the web.

kevhead75

75-90 is what ford used to specify for the rear diffs but have since updated to 75-140 as stated above.

djquik1

They switched in 2003 on the Heritage body style, if I am not mistaken his vehicle was produced after that considering it is a Lariat if his screen name accurately depicts what he owns

2004Lariat4wd

That is correct. I have a 2004 Lariat 4wd with 5.4L V8 and Auto trans. It is the "new" body style (according to the older manuals).

djquik1

If it were mine, I'd have the 75w140 in it....but it is your truck

2004Lariat4wd

I discussed this issue with a mechanic at a garage I frequent. He suspects that changing the (presumably) factor gear oil to the synthetic (Royal Purple) at 105,000 probably caused the seal to deteriorate. He claims he's seen that before. Interestingly enough, he brought up the question of the synthetic gear oil before I thought to mention it.

He further told me that the repair could be as simple as described in some of the forum articles (disconnect drive shaft, take out some bolts, pull off old seal, put in new seal) or it could be a real pain that involves removing the cover and screwing with bushings, pressure something-or-others, etc. He said sometimes it spells the end for the rear end once you get this involved in it.

His recommendation was watch the fluid level and live with it until I'm ready to open a can of worms. That isn't really the direction I like to go with my truck.

Thoughts on the synthetic theory? I'm thinking that people also tend to put in synthetic because their vehicle mileage is high and it would just be coincidence that seals would go at a higher mileage.

Thoughts on the repair? I'm considering AAMCO. I know a guy that owns one and he's an honest broker. I'm thinking they may have the knowledge to handle the problem.

alexm

Use 75w140 synthetic. When I replace those seals I pull the driveshaft, and the yoke. I then Pull the seal by screwing a drywall screw in the seal attaching a small slidehammer to the screw and slidehammering it out. I pack the new seal with assembly goo and drive it in. Just make sure you tighten the yoke appropriately but NOT over tight.