ZeeOSix

Hey Guys,

Does anyone know how the differential vent really works?

Some say it's a one-way "valve" that is somehow spring loaded and only vents when there is a pressure build up in the differential. Is that really true, or is it just normally open to the atmosphere at all times?

Some guys think that one reason differentials may sporadically leak is because the vent gets stuck closed sometimes and can't vent the internal pressure.

So, does anyone know how the differential vent is really constructed and functions?

C5FAST

Well I looked at the shop manual but you cannot tell from the shop manual how the vents works. As a rule however the vent usually allows venting of positive pressure from within the differential. If it didn't you could conceivably blow out a seal or something. FWIW.

patton

i do know. i have a spare diff. i pried the cap off there is a spring and flat rubber washer.the cap is crimped on and is supposed to be loose. the spring is very weak i would be surprised if it could hold any real psi. the vent assembly is pressed in housing but can be removed . its easier just to pop cap off to check for clog. ive had 3 vettes blow psi when checking fluid levels. i mean i was doing other things on these cars 15 mins. and they still had psi. most autos dont use spring and washer jusy loose vent cap. 4 wheelers have issue with clogged vents and ive seen many a seal changed because they didnt check the vent. enough psi and the oil will find a place to go

ZeeOSix

patton -- thanks for the info.

So it sounds like the spring is captured inside the cap (that's loose) and pushes the rubberized washer down on the vent hole, thereby making a "one way" valve.

On my Zee, I can rattle the vent cap around an push it down some against the spring, so that seems normal. I guess it's still possible that the rubber washer could adhere to the hole that it seats on and thereby cause pressure to build up.

I was thinking of devising some kind of Tygon tube to go over the vent, then remove the fill plug and suck on the Tygon tube to see if I can get airflow through the vent. Like wise, gently blowing on the Tygon tube should show no or resisted flow back into the diff. Of course, there would have to be a good seal between the diff where the whole vent assy sits and the Tygon tube for this test to work. I don't think it's very easy to get the vent cap off, and once off it may be ruined too much to be put back on.

Wonder why the Zee's diff has this setup ... maybe they thought it might be 4-wheeled.

allmee

I would like to know why the shop manuel gives NO info on the vent valve other than it could be the cause of a rear end leak, and to check it or replace it. How is either achieved? I am I supposed to guess?

patton

yes blowin in it will tell ya if its clogged. the cap comes off with a gentle pry of a screw driver. if ya got good access reinstall then recrimp with pliers. i said i doubt it [the spring] could cause it to stick. but first time i checked a c5 diff. fluid level i got sprayed in the kisser and the car had been sitting on the lift for 15 mins. or so. a 4 wheeler trick i used with great success was to remove cap and add 12 inchs of 3/8 fuel line double looped

ZeeOSix

If the vent is designed to only vent pressure from the inside of the differential, then blowing air from the outside into the vent should not flow any air. If there is a rubber washer being pushed down by a weak spring to seal the vent, then it should only vent in one direction.

I see two ways to test the vent system. 1) Put a piece of Tygon tube over the vent (has to be sealed on the diff housing which maybe difficult to do), then remove the fill plug and gently suck on the tube to see if the vent opens and flows air. 2) Remove the fill plug and put a piece of Tygon tube up against the fill hole and gently blow to see if the vent flows air out of it. I don't know if mouth pressure would be enough to over come the spring or not, guess it would be worth a try. If the vent didn't work with mouth pressure, I would be hesitant to use any compressed air pressure on this test for fear of damaging a axle seal if the vent was really stuck bad.

patton

just blow in filler hole the spring is so weak that lung psi will be enough. if you pull the cap off with washer and spring you can clean out the tube with a piece of wire. put no more that lung psi in diff no compressor air. at speed there is a lot of aeration going on gear lubes today dont foam like they used to but still build up some psi and if a vent is clogged the lube will leak at the weakest point. make sure that fluid level is 1/4 below fill car level

ZeeOSix

I decided to test the differential vent for operation as discussed above, and found that it did indeed vent when some slight pressure was inside the differential. I jacked the rear end up about 6 to 8 inches so no diff fluid would come out the fill hole, then removed the fill plug. I rigged up a hose, finding a piece that fit snuggly inside the fill hole. The hose was long enough so I could blow into the hose while reaching up and feeling the vent for escaping air. Here's a pic of the hose setup.



I could hear and feel air escaping out of the vent after blowing some slight pressure (by mouth on the hose) into the differential. It's pretty hard to build up any significant pressure with your lungs (maybe 1 psi max), so I would estimate that the one way, spring loaded vent valve started opening at maybe 1/4 psi or so. This seems like normal operation to me. If you rigged something like this up and couldn't get the vent to open by using your lung pressure then I would say the vent if defective.

chuckster

So my question is this.. If Positive pressure is bad... then what about negative pressure? Is a vacuum bad too?

What I mean is that when the diff heats up the air will pressurize and release. Then the vent will seal again.. If the diff cools down you will have a slight vacuum in there..

Same principle used in sealing mason jars.. You put em in boiling water then put the cap on.. After it cools the cap sinks in with vacuum.

Warp Factor

I guess mild vacuum isn't considered an issue. It might cause air infiltration at the seals, but at least it won't cause an oil leak.

LIGHTNING

where exactly is the vent tube located?? pic would be appreciated!

brucemoose2

I just did new gear oil in the diff on my 2k FRC. When I cracked open the fill plug (which I always do before I crack the drain, just in case...), there was an audible hiss which I can only assume was either air entering or (more likely) escaping the differential. I suspect that my vent may not be working correctly. I didn't have time to look around under there, but I plan to do that tomorrow. Where is this vent, when staring at the diff from the back side?

PS. I have a very small amount of seepage on the driver-side axle area of the diff. Perhaps my pressure issue has blown some oil by the seal...

patton

above left drive axle

pewterrag01

I've had tiny puddles of green fluid under the left side of the diff for about 6 mos. with the car level, I opened the fill port and fluid ran out.
I think most of the "leaks" that are being found are not leaks. Would there be a valid reason for all the "leaks" to occur only on the left seal? I'm going to simply check the level every time I change the oil, and i expect that it's not going to be low any time soon.....

brucemoose2

Green fluid? You might want to get that stuff out of there. I've never seen green gear oil. Maybe it's contaminated? Gear oil typically looks like regular motor oil or maybe a little grey... not green.