Vent lines

[osteo16]

So, 1989 M998, Replaced tranny and tcase and was about to reinstall vent lines.. Saw the TM for the “new” setup which I can handle fine. Was checking for obstructions in line by blowing a little air through lines. The lines definitely returned air back to my end after I put a little pressure in.. Kind of like blowing up a balloon and the air comes back out..Is this normal or should my vent lines be completely clear and able to pass air freely throughout system?? I haven’t been through the hubs or diffs so I’m wondering if they could be overfilled.... anyone ?? And Thanks

Ocho

 

[papakb]

Vent lines are supposed to vent the air in the system, not hold it captive until you release it. You should be able to blow thru them into the air cleaner.

 

[Coug]

Depending on where the other end of the lines go to. I would expect everything in the system to be sealed except for the end of the vent that's actually supposed to be open to atmosphere, i.e the actual vent portion. If you're blowing in from the vent tube then there are no other paths for the air to get out.
Blowing into it you're compressing a little bit of air in all the open spaces in the hubs/diff/whatever, so yes, I'd expect the results you are seeing.

 

[osteo16]

But shouldn't the path of least resistance take the excess air and move it to the air cleaner housing like i imagine this system would work.. Pressure builds up and air moves freely in and out of vent lines via the terminating point in the air filter housing....so if i put air in vent line, should it not escape at the filter housing??? Its acting like a sealed system... that means im probably gonna have to chase down a blockage i imagine... Anyone ??? and its Army NG truck, so i dont believe or have seen any signs of a deep fording kit..

 

[Coug]

Mine has the fording kit, so I can't verify everything.
Pretty sure though that there are two separate vent systems on this thing.
Reason I say this is there is one vent line that does run to the air filter housing inlet (not the one to the restriction gauge) and a second vent line that goes up the back of the snorkel system. And only one of the two goes to the deep water fording valve assembly. The other the lines go off under the engine somewhere.

Hopefully someone like RWH or another expert on these things comes along and confirms or denies.

If it's 2 systems it makes sense to be fully sealed and not have air escape somewhere else when you blow into it, all the vent is there for is to keep pressures equalized as temps change, not like it has to move very much air to do that, and to keep junk out. (the one in the air filter inlet is before the filter, so it doesn't have anything to keep junk out of it as far as I can tell)

 

[Coug]

okay, it was pointed out to me that the one vent just goes to the fuel tank and stuff.

On my truck the line going to the air filter housing has a little bit of a kink in it, there is always the chance there is something blocking your vent line.

 

[papakb]

When the snorkle is installed there's a small tube that runs up the side of it and connects near the top. That's the vent for the hubs, diffs, and power train. The tank vent is tucked in behind the overflow tank and terminates in what looks like a standard inline fuel filter. This might help:

 

[REF]

I can't quite make sense of this because I cant tell which line your blowing into so at the rick of sounding dumb, pull a vent line off some where else and see if you can blow air through the line you are currently blowing on. There is only one line to atmosphere and if that happens to be the one you are blowing air into then your description makes sense otherwise I would follow the vent line down from the filter at the base of your air cleaner, to the first connection and blow back towards the air cleaner. keep moving down the line until you find the blockage.
Hope it helps.