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I'm Too Late! - Rear Diff Breather Mod on 3rd Gen 4Runner

August 7, 2019.

I have a confession to make: I procrastinated this job and it may have cost me.

So let's back up for a moment. See, this was a mod that I knew I needed to do to the 4Runner as soon as I got it. Toyota has conveniently placed the rear diff breather right on top of the rear diff - pretty much as low as it can go. All well and good except that it's not at all well and good. In that position, not only can the breather can suck water into the diff when submerged in deep water, it can also get clogged up by road grime.

This latter issue isn't one that you generally hear of when it comes to relocating the breather, but it's the one that can cost you hundreds - or thousands - of dollars if it happens. Because when it does, and the diff heats up - expanding the air and oil inside - the air and oil have nowhere to go... and so they force their way out the easiest path possible - generally your rear axle seals.

And then you get a nice ring of gear oil in your wheel wells. And the opportunity to replace your axle seals, wheel bearings, and rear brake shoes. Oh, and you get to purchase a bunch of special tools, if you decide to do it yourself.

But now we're getting into another story for another post. This time, we're just going to do the diff breather relocation...so this never happens again. Oh, the trouble I could have saved myself doing this one week earlier!

The mod is pretty simple: add an extension to the breather to move it up out of the reach of any water crossing and road grime. The hardest step is finding the right parts to do the mod. Luckily, others have gone before, and the parts have been well identified. They work for all generations of 4Runners, and all generations of Tacomas. And probably a lot more Toyotas to boot.

  1. 90930-03136 Plug Breather
  2. 90404-51319 Union

You'll also need a few other odds and ends, and tools:

  1. Two stainless steel hose clamps
  2. A length of 3/8th inch fuel line (8' is plenty, I used exactly 5' to get to the fuel door location)
  3. Slot screwdriver
  4. Drill (if you're going to drill a hole for the relocated breather)
  5. Adjustable wrench
  6. A few zip ties

Once you've gathered everything, the process is reasonably straight forward, though slightly less so on the 4Runner than the 1st gen Tacoma - only because the fuel filler gasket is a rubber material instead of hard plastic.

  1. Drop the spare, to get a bit more room to work under the truck. Then jack up the rear using the hitch receiver.

  2. Clean around the OEM breather (so nothing falls in your diff as you make the swap, and then remove the OEM breather, prep the Union by optionally wrapping it with a bit of Teflon tape, and install it.

  3. Attach the fuel line to the Union with a hose clamp and run the other end to the location you'll install the new Plug Breather. Zip tie it as you go, and don't forget to allow for rear axle droop (leave some slack). Note: If you've lifted the truck off the rear wheels in step 1, you've already accounted for axle droop.

  4. If you're putting the breather behind the fuel filler door, you'll need to remove the gasket. This is tedious, but not hard. Start by removing the nut that holds the latching mechanism, and then pry out the two (2) wire clamps that hold the gasket in place.

  5. Drill a hole in the place of your choice in the gasket. I choose the 9-o'clock position since it seemed to be out of the way and have enough room to make a small hole.

  6. Push the new Plug Breather through the gasket, and then attach the fuel line with the other hose clamp. Then, reinstall the fuel filler gasket.

You're basically done with the mod at this point, and your 4Runner will thank you the next time you make your way through some water. Or mud.

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