Thought I'd post my experience from this morning changing out the differential fluid in the rear of my 2006 Titan. I bought it about 4 months ago, and with nearly 60k on the clock, I decided I would change most of the fluids between 60k and 65k, just to be safe.
For the most part, this change was easy. It looks like the previous owner liked to off-road in the titan, and I had to spend a little bit of time on the drain bolt. It looks like it had been smashed on a rock and had munged the hole so that I could not get the 3/8" ratchet on it. After digging at the hole, I was able to get it off.
As I removed the plug, I noticed the plug of nasty goop on the plug, but much to my dismay, no fluid came out. I know I had read about this on the forum, so my immediate thought was "great, I've been driving for the last 10k fluidless". I cleaned up my plug and re-installed it, and decided to try and refill it anyways.
1/3 a bottle of fluid through the fill, fluid started coming out the fill hole. At this point, something was weird, so I decided to try and remove the drain plug again. Sure enough, the diff wasn't empty, the drain area was just so plugged up with gunk, that it didn't release the fluid when I pulled the plug. Woohoo! no need for a new rear end.
The fluid that came out was clear (outside of the big chunks of goop that appeared to be gunked up at the bottom of the diff. I let it finish draining all the way, and sealed the plug and re-installed it. I then proceeded to slowly refill it, this time putting in the 2.1 to 2.2 quarts of fluid that it takes to fill. Much better.
Overall, this was an easy change. Took all of an hour (because I effectively drained and started to fill it twice). Wanted to provide this info in case anyone else encounters a similar situation. I'm guessing this was the first time the fluid had changed, but I had no noise or obscure operation from the rear end, so I'm confident that things will continue working properly from here.
It seems to be a combination of metal shavings (minor wear) combined with oil which has burned / sludge-ified. I believe some of this comes from the additives that they add to most oils (things like Teflon, graphite, etc) to provide different types of lubricity.
I'm guessing it builds up the worst in the differentials because it doesn't get changed nearly as much, and there's nowhere near the volume of fluid to dissipate large amounts of heat, etc.
That's just my guess though.
What was interesting in mine was, after all the goop / gunk was out of the bottom of the differential, the fluid came out looking a bit like honey (in color, not consistency) and seemed to be fairly clean.
While at it, also replace the breathing valve as many Titans has problems with the breathing valve and reasons to why so many Titan owners have busted rear axel seals. If you drive in water, you may want to bring a line up via the rear light. See a post by another member here under do it yourself
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