My recollection is that the EVAP (charcoal) canister is open to atmosphere on one side (originally the bottom).

When the vehicle is sitting, air pressure and temperature changes cause the fuel tank to "breathe" through the canister freely. The not-yet-burned fuel would be trapped in the activated carbon inside the canister. No gasoline smell around the vehicle any more

Now, fire up the engine. Once warmed up, vacuum is drawn against the top of the canister as long as the engine is running. This purges the the un-burned fuel from the charcoal by drawing a steady, small flow of fresh air through the charcoal and processed through the engine.

Now for the fun part: Depending on engineering choices, the tank could breath both ways though the canister (totally sealing gas cap) OR only out through the canister requiring a vacuum breaking (check valve to atmosphere) gas cap.

My 1984 Chevy Celebrity got the tank crushed because of a gas cap foul-up.

So, get out the service manual, the under hood sticker, a hand-operated vacuum pump and a clean length of 1/4" hose...

Use your own breath to blow through the 'tank vent' connection. Every other port should NOT hold vacuum. DO NOT personally suck on those connections, gasoline tastes terrible!

 

Ok so sounds like the port on the canister that the tank vent connects to should have free air flow, that's easy to test. The top two ports I assume the top actuates the purge valve and the next down provides purge vacuum.

 

I can't say which port is which since I haven't taken a Jeep apart in many years. (Now ex-)wife's 1989 Grand Wagoneer around Y2K.

But you are correct in that the tank vent line should provide a passage for vapors from the tank.

Check the specifications. My JK uses a sealed gas cap. Make sure your Jeep calls for a sealeded gas cap before you ass-u-me the tank vent line flows both ways.

 

Hopefully someone can share or help me dig up a service manual. I'm not sure the specifics or how to get them currently. The fact I do know is that my current gas cap most defiantly doesn't let air into the tank lol

 

Which Jeep are you attemping to fix. Your signature has something close to a zillion of 'em.

Is the emissions sticker under the hood still readable? If, "yes" it'll show which kind of gas cap is supposed to be there.

 

HA! yea guess what I'm working on would help. I'll edit to add that info. '91 YJ is the current project in question.

 

'91 YJ 2.5L In the processes of figuring out why my fuel gauge didn't work and my fuel pump was crying I've created one heck of a rabbit hole. At this point I'm just rebuilding the fuel system front to back with the exception of injectors (I hope). Long story as short as possible, tank isn't able to get air, so as fuel is pumped out vacuum is created which smashed my tank and bent my sending unit into 90*. Found a collapsed vent line where it goes over the frame near the gas tank, no air would pass through it. Then as I worked forward I noticed someone plugged the tank side of the evap canister. Probably trying to fix the collapsing tank issue thinking it was pulling to hard on the tank /shrug.

Since I'm putting all this back and visual inspection of the evap canister looks positive I'd like to test it to verify operation. But I don't have service manual and plugging in the 1's and 0's into my typewriter isn't turning up a test procedure for me. Anyone have an idea how how to test it or a guide on the procedure? I saw a reference to a drop box link that contained service manuals but it appears to have been taken down at some point as the link doesn't work.

Side question: I'm probably not going to use the rubber isolator the fuel lines normal go through (depending on how tough they are to get). Any ideas of what I can wrap the lines in to prevent chafing on the frame?

Here is the link to my manuals which was working 10 minutes ago.

Dropbox

www.dropbox.com

Also below in my Signature under Jeep Manuals

I haven't seen how the 2.5 vacuum is routed with the EFI, but my early 4.2's canisters were sealed, and fume's were released into the fuel intake to be burned through a myriad of solenoid crap. Aftermarket Carb install changed that into an open system, at which point I put a one way vent valve onto the vent line to the tank allowing air into tank but restricting air out. But it sounds like the problem was in your gas cap. The tank should hold pressure, but not vacuum. Get a vented cap.

 

Thanks for the link, I've saved for future reference. That should be very handy in the future. Unfortunately doesn't appear that the evap system is covered in those manuals. Be willing to bet there is a separate manual for that system.

 

it's in section 25, page 2121, of my FSM, but there is no test procedure only a description of what it is where it is and what it does.