Rear Differential- Pressure?

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Jun 16, 2018
Is it normal to have pressure buil up in rear differential? Checked fluid level in 2015 LX today. As I removed the fill plug, I could hear the pressure releasing, almost like opening a pop bottle slowly. LX was cold, had not been driven.

I checked the breather, all clear, cap moved freely. I’ve never had pressure when changing my 4Runner fluid.

Are you sure it was pressure and not vacuum?

Toyota uses a relatively advanced cap on their axle breathers. There is a shroud covering a rubber diaphragm and spring, with the rubber sealing on a lip on the axle end of the breather hose. The intent is to allow air to escape as the axle heats up, which will prevent too much pressure blowing out seals, but to some extent prevent water being drawn into the axle when it cools rapidly.. like what would happen during a water crossing.

If you are sure it was pressure it's not too difficult to unscrew the breather hose nipple from the diff and verify air flows outward. There will be a slight restriction from the mechanism I explained above. Shouldn't be able to draw much air in, though. Just clean any mud or dirt around the threads off before removing.. then seal it with some PTFE or similar thread tape.
Could’ve been vacuum I guess, sounded like pressure. The check valve makes sense. Alarmed me because I’ve not ever experienced this with my 4Runner’s. Thank you for the explanation.

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