So I finally got off my behind and did my rear diff breather mod. I had started and earlier thread on the subject where I referenced this thread on Tacoma World:

Rear Diff Vent behind the tail light Mod - Tacoma World Forums

My excitement was short lived when I realized that wasn't really a viable solution on the 5th Gen 4Runner (though I still borrowed from his suggestions).

I ordered the following parts from local Toyota dealer:

Union, part# 90404-51319
Breather, part# 90930-03136

I don't remember the exact costs, but they were cheap (less than $20).

I bought 30' of 5/16" fuel line (I have two other vehicles that need this mod). After doing a couple of practice runs I cut a piece at 9' and it was about perfect length.

Next I located the rubber grommet plug in the jack compartment referenced in @hoeftm2002 's thread:

New Location for Rear Diff Breather Ext.
@hoeftm2002 did a good job documenting locating that grommet, I'm not going to repeat it. I drilled out the grommet/plug so that I could fit my fuel line through it and re-install. Drilling it out was only sort-a successful, ended up rough cutting it w/ my pocket knife w/ a backing board:



I know, its rough, but with the hose stretching through the opening it seals pretty well. Next, I slid the hose through the grommet and fed the excess hose through the grommet hole to the underside of the vehicle:



Next I drilled a 3/8" hole above through the fuel filler rubber seal into the bracket hole behind (in the jack compartment). Hole location is just above the fuel filler door retainer clip. Push on the rubber, you can feel the hole in the bracket behind it and see it from the other side. I started w/ a real small bit and stepped up 3 or 4 times before I went to the 3/8" bit. The drill never made a clean hold in the rubber, but a stretch fit was good for my purposes.

I slipped a 5/16-5/8" hose clamp over the fuel line and gently tightened it on the line about 4" down to stop it from falling off while I put the hose on the breather barb. I pushed the barb of the breather through from the fuel filler door side and got my son to hold it while I lubed it up w/ some dishing washing soap on the jack side. My son (a grown man) pushed on the breather from the outside while I pushed the hose on from the inside. It was a real pain, space is limited! A lot of grunting later it was on. I put the hose clamp on and tightened down, again, son pushing on the breather from the fuel filler door side helped a lot. I wedged the line down and made an S turn behind the brackets for the jack/jack tools and secured the hose on the backside w/ a couple of tie wraps.







The breather is now installed in the fuel filler door area, just above the retaining clip:




It doesn't interfere w/ the door at all, is unlikely to get fuel on it here and I think it looks clean like this.

I fed the excess fuel line out through the grommet hole, pushed the grommet down and re-installed. The picture below is actually from a previous test fit, I forgot to snap one after ran the line behind the brackets but you get the idea on grommet fit (the silver thing is my flashlight):



So underneath the vehicle where those drops out there is a piece of sheet metal w/ an inviting hole ~5/16" drilled through - perfect for a clamp! I looped the hose and used a 1/2" insulated clamp (local hardware store) to attach it to the body with 1/4x1" SS bolt w/ a washer and lock nut:



Now I added a 5' piece of 3/4" split wire loom tubing (local hardware store) onto the fuel line and tie wrapped it about every 6-9". Then I ran the fuel line/wire loom over the frame



I ran the fuel line/wire loom over the cross member paralleling the fuel tank filler line. I got a wet rag and cleaned up the differential by the 1-way valve with a wet rag so I wouldn't knock loose dirt into the diff when I removed it. I removed the 1-way valve w/ a 14mm box end wrench. As others have mentioned, there was a hiss as air rushed into the diff (it was under vacuum) as it was removed.

I installed the Toyota union w/ a 13mm box end, teflon tape on the threads. Once the union was installed, I put a 5/16-5/8" hose clamp over the fuel line, lubed the barb with dish washing detergent and pushed it on. Again, tight spot, lots of grunting, its on now... Tightened down w/ the hose clamp, cut off about 1" excess wire loom tubing and tie wrapped wire loom to fuel fill line leaving a generous S in the line to allow the diff to drop/wiggle. The whole install looked very clean and professional till my son decided to add a few white nylon tie wraps (I think he does things like this just to make my eye twitch...):





Then we jacked it up to make sure there was plenty of extra - there was:



Glad I decided to jack it up - the jack wouldn't extend high enough w/ the new lift/tires! 2x4 was all it needed, cut a piece, wrapped w/ an old shop towel to muffle the noise and dropped it into the jack compartment for when its needed.

David

Realized its hard to see much on the pic of the breather in the gas fill area. Took a better one w/ my real camera:



David

Nice work!

Looks good! Good job!

What is the purpose of the rear diff. breather mod? Does it have a tendency to overheat when used offroad?

It's so when you have your rear diff under water it wont suck in water. Same concept as a snorkel.

The front diff, transmission, transfer case, rear diff (and rear e-locker if you have) all must be vented to allow for gas expansion as they get hot.

The front diff, transmission and transfer case have vents that are routed to the fire wall. They are not real high, but you won't get water in your case unless your are running through real high water/mud.

The rear diff doesn't have a breather. For some reason, Toyota just installed a 1-way valve on the rear diff which allows gases to expand out, but not to contract when cooled rapidly. When you are driving around and the rear differential gets hot from normal driving and you splash into some water the sudden chill from the cool water causes the case to rapidly cool and contract, building a vacuum in the rear differential.

This sudden quick cooling can create a strong enough vacuum that it sucks in water through the rear diff seals. The rear differential does NOT have to be underwater to have this happen - water just has to be up to the level of the seals (roughly about a 1' deep if I remember correctly, I didn't measure). Obviously, a little bit of water in the rear differential is gonna cause some serious problems if it stays there for long!

This is not the sort of thing that happens every time you splash your rear diff. I drove my '07 through some deep water (about 15") many years ago without the rear diff breather mod and had no problem. Eventually though, it WILL happen and it COULD happen the very 1st time. The rear diff mod eliminates the 1 way valve and vents the rear diff to a higher point.

Oddly enough, Toyota did put a proper vent on the e-locker actuator on rear diffs that are so equipped and that is run up higher to the under side of the frame cross member. If you are back there and running the vent for the diff you may as well tie the e-locker vent in as well.

David

I realize this is an older thread, but my question still applies. How exactly does this relocation help since the 1 way valve is still in place, only higher? Won't a negative pressure still cause water to be sucked in from the rear diff seals?

#1). The long length of hose probably provides enough flexibility to compensate for the pressure differences, maintaining seal integrity.

#2) #1 doesn't matter because I replaced the one way valve w/ a breather, same part number as the breather the front diff. Check the parts list in the original post...

David

Ah, I see that now, thanks!

I bought the parts and some hose a few months ago but was not sure I really needed it. Maybe I just got lazy. Guess what's going in the next few days?

I have been through a few water crossings, so I better change the diff fluid while I'm under there getting dirty and crawling around on the garage floor.
--- Glad this old post popped up.

It is an insurance policy - you may not ever need it. Just because you dunk your diff doesn't mean you'll get water in there. In a bad flood in Houston I had to drive my '07 Tacoma through about a mile of water 8-15" deep. I did not have the breather mod at that time and I was certain I got water in there. Brought it to have it serviced and they said the diff fluid looked brand new.

Conversely, one of the guys I wheel with dunked his rear diff in a creek crossing w/o the breather (about 15") and his diff fluid was milk! You just never know for sure what factors are going to combine to make it suck some water. The breather mod buys you a little more piece of mind.

David

I’ve done this mod before, just getting to it now on my T4R, since my parts came in.
Some people think/say it is unnecessary, and that’s their opinion which I respect. Some people that haven’t done it, in hindsight, wish they would have.
We have referred to it as the “Scuba Mod” (reference Toyota FJ Cruiser Forum - View Single Post - Saker's 2013 TT/SE Build) and I did it on my FJ Cruiser.
This one being done is just slightly different because of how it is routed and configuration of the bottle jack compartment. I used @hoeftm2002 ’s example in his post: New Location for Rear Diff Breather Ext.
Cut the hole with a hole punch in rubber plug # 90950-01958, inserted fuel line, routed it, removed OEM one-way valve from differential, installed the 90404-51026 Toyota 1/4" Fuel Line Union, left enough slack in fuel line for rear end flex and installed line to union with clamp. Cut fuel line at bottle jack compartment to correct length, installed line to 1/4 X 1/8 HB Male Adapter with clamp, installed the bracket to bottle jack compartment. Done!
Photos included with descriptions below.
Parts used:
(1) 90404-51026 Toyota 1/4" Fuel Line Union (Cost: 6.00)
(1) 1/4 X 1/8 HB Male Adapter (1/4" Hose ID X 1/8" Male NPTF Adapter)
(1) FJNewb 2 Port Manifold with Filter (Cost: 30.00 Shipped) Just the Manifold/filter can be purchased!
Link:Differential Breather Kit - Raw Mind Engineering
(1) 1/8" Brass Pipe Plug (Already had)
6' of 1/4" Fuel Line (Already had)
Hose Clamps to fit. (Already had)
Aluminum Bracket fabricated: I made mine out of a piece of 1/8" thick aluminum cut to size, bead blasted, marked & drilled the holes. (Already had)
I used brass machine screws, washers (flat and lock) and nuts to attach manifold to bracket. (Already had)

Scuba Manifold w Bracket


Differential Nipple


Diff Check Vv


Hole in Plug cut w 1/2" Tubing


Hose & Plug Attached


Hose Routed


Hose Routed On Fuel Fill Line


Nipple Installed w Fuel Line


Flexed And Still Slack In Line


Scuba Maniflod Mounted In Jack Compartment

Just did mine too! Thanks for the write up!

This is for all you guys with a TE right? The TE is the only model with a rear differential.

Am I safe to assume the SR5/Limited would not need this mod? (I believe the LE only has a center diff, not sure about the SR5)