Diff Breather

I've seen this topic discussed in other posts about seals, bearings, breather mod, so I wanted to contribute the following. Replaced my diff breather today, 2006 165k, pretty sure this is the original breather. I don't have any seal leakage yet, but my truck sees winter salt in Maryland so I thought it wise to replace.

After driving 40 minutes on the highway, truck cooled down for about an hour, but the housing was still warm to the touch.

Here's what was interesting, when I unscrewed the existing breather there was an audible psssst of air for 1-2 seconds. I thought, pressure in the diff, vent was probably clogged, good thing I changed it.

Here's the disassembled breather:

No corrosion of note:






Here's the spring pressure according to Thomas Jefferson:






Since my original vent has no corrosion, let's assume that it was functioning perfectly. If so, I'm wondering if the Toyota designers intended for the axle housing to run at a positive pressure on purpose.

Perhaps the designers counted on the few extra psi inside the housing to actually help the seals mate better to the axles. The extra pressure helps squeeze the seals to the axle better than just relying on the mechanical properties of the seal alone.

That might explain why the sophisticated vent instead of a barb fitting and 18 inches of hose like GM and Ford trucks get from the factory. Kinda like the SR-71 fuel tanks that leak on the ground but swell shut when heated at mach 3.

 

Did you try twisting the cap on the old breather before you disassembled it? The cap should be free to move if it's functioning correctly. A couple years ago I changed my rear diff fluid and when I took out the fill/vent valve I noticed the loud pssst of pressure build up. I found the breather was caked full of mud and dirt. Replaced it but the damage was already done - my pinion seal was leaking.

 

Mine also made a hissing noise when I removed it years ago. I suspected it was vacuum since the valve is a one direction style. I figured when the air in the housing heats up and expands it meets atmospheric pressure (give or take) and when it cools down and air compresses it cannot inhale through the valve, creating a mild vacuum.

 

History and science! I love it.

I had some rear seals go bad and the shop did not replace the rear breather. After learning on this site about this breather sieze problem, I immediately replaced mine with a factory OEM.

The action between the new and old was night and day. The old still allowed air out, but at a lesser rate. The old cap was sticky on certain turns with the hand and leads me to think regular road driving could bouce it into a stuck position?

I did use some 3/8” gas rubber line, as an extension for the new one, lead up to/and behind the gas cap door.

I do think the negative pressure ‘psst’ you are hearing is inside due to the cooling effect. Open the housing and air ‘sucks’ in to equalize.

 

That's why i disassembled this one. I don't think twisting the cap is an indicator of proper function. I should have taken another pic but the vent assemble from top down goes like this

Metal cap
Spring
Rubber disk/seal
Threaded barb with hole
Axle housing

_____Top of cap
[ s ]
[ p ]
[ r ]
[ i ]
[ n ]
[ g ]
____ rubber disk/seal
* *
* *
* *Threaded fitting with hole

So if there is a bunch of dirt and crap built up on the spring, twisting the cap will kinda loosen it, but twisting the cap does little to dislodge or affect the rubber disk threaded fitting interface which is where the venting takes place. But if dirt/corrosion was preventing the spring from compressing then yes.

 

Used to watch SR's leaking fuel taxiing out to the runway for takeoff at Kadena. Kind of weird but quite a unique bird.

 

Darkness and Hand,
I'm of the opposite understanding. When the axle heats up, the air in the axle housing expands, the expanded air, once the internal psi of the air exceeds the spring pressure, lifts the rubber disk and pushes out, so the housing is always at positive pressure relative to the outside air due to the spring. Since my housing was still warm, the psst had to be a venting of pressure from inside the housing, not psst of air from outside going in. We need an engineer thermodynamic member to confirm.

 

It's a pretty shitty design. I dont think they intended for it to be under pressure, but equal to outside. Which is odd. I dont see why when it gets hot it doesnt compress the air, but manages to force it's way past a seal.

 

Not atmospheric pressure, that would be an open tube from housing to outside air. The spring vent forces the housing to build pressure, which then vents, only once the pressure is high enough to push the spring. After it vents, the spring slams the rubber disk shut before true equilibrium is achieved thereby maintaining a slightly higher pressure in the housing.

Maybe the intent is to have positive pressure to keep water out. Any water that makes it to the axle seals or pinion seals will need to be under pressure or force to overcome the positive pressure maintained by the vent. This way any residual water that splashed or washes over the housing wont get in. With a system in equilibrium, the water has a greater chance to seeping if, if only due to gravity. Think of the biological protection on an Abrams tank. The interior of the tank is maintained at atmospheric overpressure so any chemical or biological agent cant get to the tank crew cause any hatch or seem has a constant stream of pressurized air being pushed out.

 

You can only blow through the vent from the threaded side it will not suck in. I tested this theory when I relocated my breather. The best thing to do would be relocate and put a proper ARB breather on there. Those allow air in and out. It’s more of a filter. The way the breather is designed it will pretty much suck water in your axle seals, instead of sucking air or water through the vent if you go through a water crossing. Poor design

 

I have a scenario, tell me if this is plausible.

Drove an hour, pulling a boat to the boat launch. Backed down the launch with a hot diff at a funny angle at the water's edge (because the launch is curved) causing the diff and driver's side rear wheel to be partially submerged in cold lake water (at least up past half way on the wheel). Two weeks later, driver's side axle seal is leaking. Did I cool the diff too fast and cause a lower pressure which in turn sucked in water from the lake through the driver's side axle seal?

 

Definitely sounds plausible.

 

Right, I couldn't think of a way to say a stabil balanced pressure for the temperature, but you are correct that it is not atmospheric. My thought was that since it only breathes out, once that temperature drops the pressure will drop with it and create a mild vacuum.

 

I have a plastic microwaveable container with a rubber seal around it and 4 locking tabs to keep the lid on. I heat up my lunch in microwave in said container AFTER UNLOCKING the 4 tabs to prevent explosion. The lid slightly rises as the food heats to hot.

I remove the container and let sit on counter top to cool off a bit. Lid has FULL suction at this point a few minutes after being super heated. I can’t remove lid without wedging/prying a fork into the seal. A slight vaccum has happened in this senario.

Normally, at room temp (without ever heating food) that seal is easy to open.

Anyone hungry?

 

I'm heading to lunch right now

 

Mmm enchiladas

 

Sorry Hand, apples and enchiladas don't apply here. When you heat your food in a closed container, yes you are heating and expanding the air trapped in said container. However you forgot to account for the water that is trapped in your food that gets released upon heating as water vapor. So now below your lid you have the original air heated and expanded, plus you've added heated steam. So your lunch tupperware expands not just due to air but the additional h2o molecules that you've heated and released. Further, when you set it on the counter and it cools slightly, the contents are not "sucking the lid into the container" but rather, the atmospheric pressure, that invisible column of air from the tupperware lid to 95,000 ft up has weight, 15lb/sq inch that is actually pressing down on the lid, and pushing it into the container because the expanded water vapor has now cooled and lowered the internal expansion pressure.

Our axle housing does not have enchiladas releasing water vapor every heating and cooling cycle.

Also, I eat pizza.

 

Poor Mrs. Darkness.

 

Heat expands oil. Heat also causes release of vapor. The space inside the diff is finite. The heated oil expands into the finite space and also releases vapor (hence the need to ‘breathe’). The plug goes one way out. The cooling oil shrinks leaving the finite space thats not oil ‘bigger’ thus creating the ‘vacuum’.

A minature atmosphere is inside the diff. Equilibrium is achieved when the diff is opened OR enough time passes that the axle seals and small cracks (wherever) allow the equilibrium over time.

 

Finally got around to painting the cab weatherstripping which I caulked last fall.

 

Opps.

 

Not sure I agree that the heated oil behaves like water vapor. Most auto oil, and I would think particularly gear oils are designed by chemists to not undergo a phase change from liquid to vapor. But I'm not a petroleum lubricant engineer.

As to the heated oil expands, yes. Heated oil pushes existing air out of the housing as oil volume increases, yes. Oil cools and contracts creating a vacuum, maybe. I think the cooling contracting oil allows a larger volume inside the housing, and that larger volume has a lower pressure than the atmosphere of 15psi. So it's not a vacuum sucking the air in, but atmosphere pressure pushing it's way in to equalize within the housing.

Potato vs potatoh perhaps. Hopefully there's a PhD in thermodynamics that reads these post and can educate us. Till then, I'm going with genius Toyota engineers wanted the housing slightly over-pressurized to keep pesky water from seeping past the seals. And pizza, lots of pizza.

 

The heated oil expanding (and creating the positive pressure) is how they engineered the diff system (I am not arguing against this fact). In fact, its why I put a new breather on the rear diff after my axle seals blew. My old breather was stuck and the seals blew. I think we are in agreement on this? The pressure has to go somewhere. We hope it exits the breather (at its designed outflow rate) as designed.

The area which we seem to have problems is where once the oil cools down to ambient outside temp. What happens when a one way closed valve only lets air escape, but not re-enter a system after the heated oil expands and then contracts back once cooled? Answer: It creates a vacuum.

If you had an open breather (two way), then no vacuum.

Why did they design it only a one way breather? To help the seals mate per your original premise? I have no idea, but it makes sense.

In dannos case, the vacuum effect was rapid. The cold water flash cooled the expanded heated oil creating a suction at the axle seals point of least resistance (vacuum flash).

Im sure if you opened your diff plug when it was hot it would blow out (if your breather was stuck shut). If you opened it when cold (shortly after being heated) it would suck in.

Study up on the Laws Of Thermodynamics. They are easy to understand. Someone smarter than us figured them out and wrote them down for us.

 

Oh shit! It’s the first sighting of @Professional Hand Model.

 

Maybe the resistance on the spring is to prevent too much vacuum from being created by allowing a bit higher pressure to be retained. Don't we have an "ask the Toyota engineer" thread? Would be interesting to read what they say.

I tossed mine years ago and ran a hose up in the bed with a small free flowing filter on it. Mine is always at atmospheric pressure and my seals are fine.

 

This discussion has me wondering about going to an equalibrium state.

Follow me:

The breather breathes out. Where does the air come from to fill the vacuum once cooled?

The seals are the weak point. But is the slight vacuum (when diff is cooled) designed for some negative pressure on said seals? To keep oil inside the diff?

Seems and equal state of balance is a safe route.

But back to Remintons OP Premise: Toyota designed the system this way to help the seals mate.

I think they are designed to work under both slight positive and negative pressure (within limits).

 

Could be true, but I threw mine away