Charcoal canister purge valve question

[howzzzit]

How might I check the purge valve on my 72 C10 charcoal canister to verify if it's good or bad? Another question is why is it needed if one of the lines is connected to the pcv line? Seems like it would constantly be purging the gas fumes. I also think I read the purge valve should be connected to ported vacuum? Right now the only thing connected to the canister is the 3/8 fuel line. I don't smell gas fumes, but would like to hook it up correctly. Thanks.

[Steeveedee]

https://www.google.com/search?q=72+c...hrome&ie=UTF-8

[dmjlambert]

The basic idea as I understand it is:

• When vacuum is not applied to the purge valve, the connection to the PCV valve outlet is closed. So you should be able to leave the purge valve disconnected and verify you can't suck on the PCV hose outlet on the charcoal canister and get any air, or not much air. Then when you open the valve by applying vacuum to the purge valve, you will be able to draw air from the PCV hose outlet on the charcoal canister.
• If you connect it to ported vacuum, your charcoal canister is out of the circuit at idle. This keeps your charcoal canister setup from giving a vacuum leak at idle, so keeps your idle smooth. Drawing fumes from the charcoal canister and burning them is what happens when your driving down the road or other non-idle conditions.

If your purge valve is not working and the PCV port is wide open all the time, you can just plug the purge valve port and get an external purge valve and hook it up inline with the PCV port.

[howzzzit]

Thanks DJ. I'll check it out tomorrow if I can find my mighty vac. Instead of teeing into the pvc hose what are your thoughts about connecting to another manifold vacuum port on the carb? Smaller fitting than pvc, but since I'm not smelling fumes now it might be ok.

[dmjlambert]

I have a 69 with no vacuum canister and I rarely smell fumes even parking in the garage. The vapor system probably gives just a very marginal advantage, barely anything. It would probably be OK to suck the fumes out of the canister through some other vacuum port, it just wouldn't be set up according to design. If you have 50 year old activated charcoal in the canister, it may not be doing anything for you anyway.

[Rich72C10]

I have to disagree with: "very marginal advantage, barely anything." Or at least when my '72 Charcoal canister wasn't working. The vapor smell was pretty bad in the garage and into the house if I left the door opened.

After getting mine replaced/fixed (it was damaged) and then a recharged one installed was like day and night - meaning I went from having fuel smell to zero. So for me it was well worth the effort of getting it figured out.

Here is another link, which this helped me recharge a Canister. Unless you are very lucky to get a NOS one, you will need to recharge it.
Vapor Canister Rebuild - A new how to

Interestingly I have a 1969 AH Sprite in the garage now and it doesn't have a canister. The Sprite isn't causing a fuel smell. Makes me wonder if you have a car that is designed with one and it isn't working/disconnected you'll get vapor smells vs a car that never had one.

[dmjlambert]

Yes, I meant comparing to a 69 just plain vented cap gas tank, a properly working whole vapor system probably gives just a very marginal advantage, barely anything. It is more stuff to maintain, and if something goes wrong with it there will be a stink. if I were inclined to add the vapor system to my truck or repair a vapor system, I would get a new canister, which is fairly cheap and does not have the built-in purge valve. I would then just get an external purge valve and hook it up.

[howzzzit]

This canister on Amazon has purge, tank, and air ports. (thinking the air port would be pvc) Looks like I could remove the cap on the air port and add an external purge? I've emailed Dorman. Anyone tried something like this?

[71CHEVYSHORTBED402]

Nice write up, bookmarked. I smelled gas in my garage recently, may have to rebuild the can at some point. Its early take off, but

[71CHEVYSHORTBED402]

Originally Posted by howzzzit

This canister on Amazon has purge, tank, and air ports. (thinking the air port would be pvc) Looks like I could remove the cap on the air port and add an external purge? I've emailed Dorman. Anyone tried something like this?

Sure would be great if thats a replacement option

[howzzzit]

DJ I can suck air through the pvc fitting when there is no vacuum on the purge and line disconnected. I pull 15 inches of vacuum on the purge and it holds. Pvc still open.
Would this purge valve be the one for the pvc line? I assume the purge valve is connected to the pvc outlet inside the canister?

[dmjlambert]

Originally Posted by howzzzit

DJ I can suck air through the pvc fitting when there is no vacuum on the purge and line disconnected. I pull 15 inches of vacuum on the purge and it holds. Pvc still open.
Would this purge valve be the one for the pvc line? I assume the purge valve is connected to the pvc outlet inside the canister?

I can only give opinion based on my limited knowledge, I have a 69 and only have studied this vapor stuff when I have considered adding some vapor control, but lost enthusiasm for it when I saw how much extra money it was going to cost, at least a few hundred $$ for gas tank, sending unit, vent lines, gas cap, hoses and hard lines, canister, purge valve. Others here on the forum may have more experience with this. But here goes because I like to type, I'm too tired to go back out to the truck and crawl up under that dash for my current project.
I think that purge valve would be the one to get to operate external of the charcoal canister. It would connect to the PCV outlet on the outside of the canister.
I believe the canister listed on Amazon is what you would need, and connect the PCV outlet on that to the purge valve, and on the other side of the purge valve tee that into the PCV line. The tank port goes to the gas tank vent, and the air port just sits there are vents or sucks air and it is not designed to fit a hose.

[Steeveedee]

^ Must be a different design than I'm used to. IIRC, the ones for our vehicles had a filter on the bottom of the canister that needed periodic replacement. Connections were to fuel tank, purge line and vacuum.

[dmjlambert]

Yes, that type is no longer made.

[dmjlambert]

If you're working on it I recommend ordering the purge valve first and check it to see if it behaves like I described, because you never know I may have it all wrong. During my research I didn't find any really good detailed descriptions of how the system worked and had to make some assumptions.

[howzzzit]

DJ there isn't a pcv port on the canister. Just the capped off air, fuel tank, and purge. It almost looks like the air is capped and might come off. If it did wonder if I could use that for the pcv connection. Probably just pulling fresh air through the canister and that's probably what the pcv fitting does. Your thoughts?

[dmjlambert]

The purge port is the PCV port. The air port is probably equivalent of the original canister, having that air filter on the bottom to vent the canister, and should be left as it sits. That is my guess.

[Rust_never_sleeps]

Here's how the '70 CA model (LS9) was plumbed:

both the skinny lines in the circles go to the canister.
IIUC:
The port on the right sits higher in the carb, so that's the bowl vent.
The fat PCV port sits under the blades, so that'd be manifold vacuum and would draw fumes from both the canister "return" hose and the driver's valve cover into the carb.

Mine is also the type with the 12k mile filter replacement. Filters are available from Rockauto, but the canister itself is not.

Hope this helps

[howzzzit]

Ordered the ACDelco purge valve. Film at 11.

[howzzzit]

The Delco purge valve will work. Smaller than the pictures and they don't say the line sizes. The diaphragm vacuum nipple is 3/16 and the through line is 5/16. With no vacuum the 5/16 through line is closed and opens at approx 5 inches of vacuum applied to the 3/16 line. I'm gonna make it work with the available 2 port canisters. Just tie the 5/16 into the pcv line and the 3/16 to ported vacuum on the carb. I hope. Thoughts??

[dmjlambert]

It sounds reasonable to me.

[howzzzit]

Why couldn't I skip the purge valve and connect from the port on the canister to ported vacuum on the carb? It just wouldn't involve using the pcv tee. Thoughts anyone?

[dmjlambert]

Originally Posted by howzzzit

Why couldn't I skip the purge valve and connect from the port on the canister to ported vacuum on the carb? It just wouldn't involve using the pcv tee. Thoughts anyone?

I don't know why they designed it the way they did. It could be ported vacuum only works well to control valves and actuators that do not leak. I don't know if it is a concern that air going into the small ported vacuum port is clean and limited to a very small volume, while air going into the large manifold vacuum port used for PCV is OK to have gas and oil vapors in it and leak in at a steady stream.

[howzzzit]

Checked with the manufacturer of the canister to make sure the purge port isn't a purge valve. It isn't so DJ using the external purge valve is the way to go.
Checked the canister purge port on the quadrajet today and it is ported vacuum. Excellent!
Thanks

[Rust_never_sleeps]

Not sure why they'd use ported vs manifold for evap, but for PCV, I'd think you want that port below the blades to keep the oil vapor from condensing and gumming things up.