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I may have a blocked vent tube from the rear diff. I can see it leaving the top of the diff, but can't find the other end of the vent tube on a 2006 Rav.
 

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I don't know how RAV vent tubes work, but on my Jeep Wrangler the rear differential vent tube ends near the top of the passenger's side rear wheel well.

It has some kind of check-valvish cap on the end.

It could be that Toyota did something similar on the RAV, to minimize the possibility of water getting in the differential, so you may want to look in the area of the rear wheel wells.

.
 

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I think there is a tube connected to my vent cap. I know my rear differential had some work done by the previous owner so the dealer must have attached a tube to the vent cap. I can't see the other end on mine but I highly doubt that it can get plugged up unless one drives on muddy roads frequently.
 

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Toyota do not come with a vent hose. At least not in the lower 48. Even the tacoma and other more popular "offroad" models still come with a vent cap. Here is a picture of our rear diff and the cap.


The vent cap is a simple one-way valve that lets air out but not in (a little in). The idea is that this prevents water from entering but allows for hot expanding gases within the diff to escape. This is problematic for a couple of reasons.
-gunked up after a few years and prevents any venting. The result is typically a leaking diff seal; axle or pinon.
-They allow hot gases to escape, but not cool air back in. When the diff cools often a vacuum will develop within the diff. This can lead to seal leaks
-The vent gets gunked up and no longer stops water from entering.

A popular mod is to remove the vent cap, add a barbed nipple and run a hose up to a higher location. You would need to remove the stock vent and replace. You can not simple add a hose to the top of the vent cap. You can pry the top of the stock vent cap off, remove the spring and seal disk and attempt to hose clamp a hose to the remaining nub, but it is very small surface and hose pops off easily.

Both Nissan and Toyota sell nipples that thread onto the diff. About $20 said and done. The thread pitch is a little uncommon. 1/8" BSPT. NOT NPT!!! It is only 1 thread per inch difference (you can not even tell the difference on a thread pitch gauge) between the 2 but it will wreck your diff threads; ask me how I know. A 1/4" barb on the other end works well. If anyone wants one I bought a whole bag from overseas. At the time it was very difficult and expensive to find stateside; at least locally. I still have a bunch if anyone wants one;$5 shipped. PM me or email at [email protected]



Though I do not have this mod done on the RAV I have on mine and many other trucks. I run the hose up to the tail light area and added a small engine gas filter (again I have a big bag of these too) to keep dust and bugs out of the line.

Attached is a picture of the stock vent cap with the cap/spring/seal removed. This is the small surface I tired clamping a hose to. Below with the electrical tap is a 1/8" NPT fitting. Finally, at the bottom, the right 1/8" BSPT fitting.

Back to the OPs question. If you can not find the top of the vent hose you should be able to pull the end off the diff and blow into it to see if it is clogged. Doubt it though. THat is the reason why these things are vented with a hose. It would be easy to route the vent hose into the cab, but I would not do this. You will get gear oil smell in the car.
 

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Toyota should have used a hose to relocated the breather cap (higher location) in the first place so there would be less problem with it rusting and plugging up causing leaking seals. Being able to easily check the breather cap is difficult. I see on the 4runner forum where someone rerouted their breather cap to the interior behind the rear panel. I wonder if it is even possible to remove the breather cap without taking down the rear differential?
 

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Toyota do not come with a vent hose. At least not in the lower 48. Even the tacoma and other more popular "offroad" models still come with a vent cap. Here is a picture of our rear diff and the cap.


The vent cap is a simple one-way valve that lets air out but not in (a little in). The idea is that this prevents water from entering but allows for hot expanding gases within the diff to escape. This is problematic for a couple of reasons.
-gunked up after a few years and prevents any venting. The result is typically a leaking diff seal; axle or pinon.
-They allow hot gases to escape, but not cool air back in. When the diff cools often a vacuum will develop within the diff. This can lead to seal leaks
-The vent gets gunked up and no longer stops water from entering.

A popular mod is to remove the vent cap, add a barbed nipple and run a hose up to a higher location. You would need to remove the stock vent and replace. You can not simple add a hose to the top of the vent cap. You can pry the top of the stock vent cap off, remove the spring and seal disk and attempt to hose clamp a hose to the remaining nub, but it is very small surface and hose pops off easily.

Perviously I offered to sell a barbed nipple fitting, however I have since done more research. It appears that the vent on the RAV for both the rear diff and the transfer case are fixed. Possibly pressed in place. The base of the vent is round and smooth. See the picture above. I confirmed this last night on my RAV. From picture searches the from Transfer case is the same.



Thus the threaded vent nipple that works on the solid rear axle of the tacoma/tundra/4Runner/FJs... do not work on our RAV. :frown

Not sure I would be willing to pop off the cap of the stock vent and try to fix a hose to it.
 
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