280z Fuel Vapor Canister - How to Fix

[matt90gt]

I have not seen a fix for the OEM vapor canister on the 280z posted.  I found a way with new parts to still use the OEM one and have it functional.

Back ground, I have a 77 and have been chasing running issues.  I was focused on vacuum leaks due the a poor idle and erratic performance.  I found the vapor canister as culprit because it never closed the purge line at idle and caused a large vacuum leak. I wanted to keep the canister to not have gas venting from the tank when sitting.   Check out this video on the canister for knowledge and how to test:

 

The vacuum leak was from the line labeled "purge" -  since that goes into the intake and the stock purge valve did not work.  The smaller line on the top labeled "dist vac" comes from a T in the front of the TB and T's also with the distributor vacuum advance (1977 model) - this only gets vacuum when the TB blade is open, which then will open the purge valve to allow the vacuum to purge the fumes accumulated in the canister.   I actually had purchased  another used canister and it still had the same vacuum leak issue.  Instead of chasing used parts or retrofitting from another vehicle, I wanted something new and easily available.  I found an external vent valve to use to solve the issue. This would take place of the original unit on top of the canister.  This along some sealer/gasket maker is all you need (assuming your vacuum lines are in good shape):

$22 on Amazon for the Dorman unit.  This is closed by default until vacuum is applied to the top port (just like OEM)

You add the Dorman unit between the engine and canister:

Then for the top of the old purge valve, under the cap is a rubber diaphragm that seals or allows vacuum to pass (which was inoperative). Remove the internals, seal the top cap with sealer/gasket maker along the edge and use a vacuum cap for port.

Internals under the cap for reference:

Hope this helps others

 

Summary Recap:

1) purchase external purge valve

2) install between canister and intake.  + take dist vac line from the top of the canister purge valve and plug into new external valve

3) Remove top cap on the canister purge valve.  Remove internals, seal the cap back down to the canister and plug the port with a vacuum cap

Edited September 21, 2023 by matt90gt
clean up

[Captain Obvious]

34 minutes ago, matt90gt said:

You add the Dorman unit between the engine and canister:

Then for the top of the old purge valve, there is a rubber diaphragm that seals or allows vacuum to pass (which was inoperative).  I used a self tapping screw with some sealant on it to block any air entering the stock purge valve  (I didnt want to trust the old rubber diaphragm to seal anything):

That is all it takes.  Works like OEM.  I replaced the cap on the canister to make it look more OEM after I removed the diaphragm so the screw would fit under the cover

I'm really sorry, but unless I'm missing something, this modification does not work.

Sure, it will get rid of your previous vacuum leak, but only because you have effectively plugged all of the hoses. You could have just capped off the vacuum lines at the throttle body and the intake manifold with rubber plugs and the end result would have been the same.

[Captain Obvious]

But I don't want to come off as if I'm poo-pooing the idea. I actually think it's a pretty neat concept to a common problem. So... How would you utilize that Dorman purge valve to accomplish what you were trying to do?

Take the screw back out and silicone the cap (with no diaphragm) onto the canister so it doesn't leak.

Maybe use the original diaphragm along with silicone to help with the cap sealing, but cut a half inch hole in the center of it first. You want the cap to seal to the canister but you do not want the diaphragm to be able to block off the little hole where you had the screw.

One other detail is that you have ruined the metered orifice in the canister by running the screw in, so you would need to use the spare can you picked up instead of the one you had the screw in.

So bottom line?  Replace the can with the spare (with no screw this time) and put the cap on in some fashion such that it completely seals along the perimeter, but cannot and will not ever block off the hole in the post in the center.

[Captain Obvious]

40 minutes ago, Captain Obvious said:

I'm really sorry, but unless I'm missing something, this modification does not work.

Sure, it will get rid of your previous vacuum leak, but only because you have effectively plugged all of the hoses. You could have just capped off the vacuum lines at the throttle body and the intake manifold with rubber plugs and the end result would have been the same.

Actually that's not exactly the case either... If your cap seals to the can now, you have effectively capped off all the vacuum lines.

However, if the cap does NOT seal to the can (which is what I suspect), you have now introduced a vacuum leak of unknown volume when you are at light cruise on the gas pedal. The size of the leak will be determined by how well the plastic cap seals to the canister. And I suspect that cap seal is not that good since you took out the diaphragm material (because it interfered with the screw head).

You will get good idle performance because the purge valve is closed, but at partial pedal position, when the purge valve is opened, you may run way leaner than designed. Not a good situation.

OK, I think I've given this enough thought for today.   

[Patcon]

I think Captain Obvious is pointing out, it can't "work like stock" if you blocked the port with a screw...

[Captain Obvious]

35 minutes ago, matt90gt said:

The problem is like in the video  The OEM purge valve is inoperative and causing an open vacuum at idle.  Added the external valve solves this issue and works like stock being vacuum operated. 

Adding that extra valve the way you have done it does not "work like stock". It does nothing functional at all with respect to vapor mitigation, and in fact, could actually cause engine damage.

The stock system does this...

The fuel vapors that come off the tank and temporarily stores them in the carbon canister. Then under certain circumstances while the engine is running, those vapors are pulled back out of the carbon canister and disposed of by burning them in the engine. That's what the stock system does.

The system, as you have modified it does this...

The fuel vapors that come off the tank are stored in the carbon canister. Until the carbon in the can becomes fully saturated and cannot hold any more vapors. After that point, any vapors that are pushed into the tank are simply vented to atmosphere through the fiberglass filter on the underside of the can.

Also under certain circumstances while the engine is running, the purge valve will be opened and an unknown and unpredictable amount of fresh air will be pulled into the engine leaning out the fuel mixture an unknown and unpredictable amount.

That's what you have now.

[Captain Obvious]

8 minutes ago, matt90gt said:

All i have done is remove the purge valve from the top of the canister to an external unit.  It functions just like stock.  Your implication that "in fact could actually cause engine damage" is comical.  If I run the canister as it sits, it creates a vacuum leak which will lean out the fuel mixture.  That will cause damage.  By adding the external purge valve, it cuts off the line until the butterfly in the TB is open, which will create vacuum at the purge valve and open the line - just like factory. 

OK, then let's start simple....

Why would you want to have the purge valve open? What would be the reason to ever do that? Explain to me why having the purge valve open is a good thing.

[matt90gt]

3 minutes ago, Captain Obvious said:

OK, then let's start simple....

Why would you want to have the purge valve open? What would be the reason to ever do that? Explain to me why having the purge valve open is a good thing.

I assume you are talking the aftermarket unit?  if so, it works like OEM where it is closed until vacuum is applied to the smaller line there.  Maybe that is the confusion here.  Again, works like stock.  closed on the vent line until vacuum is applied.  Since the stock unit is inoperable and leaving the vent line open all the time (causing the vacuum leak) I replaced it with an external unit for $22.  Cheap fix to the OEM unit which is still doing its job filtering.  

Edited September 19, 2023 by matt90gt

[Captain Obvious]

Just now, matt90gt said:

I assume you are talking the aftermarket unit?  if so, it works like OEM where it is closed until vacuum is applied to the smaller line there.  Maybe that is the confusion here.  Again, works like stock.  closed on the vent line until vacuum is applied.  Since the stock unit is inoperable and leaving the vent line open all the time (causing the vacuum leak) I replaced it with an external unit for $22

I was actually talking about either the stock purge valve, or the aftermarket one you put in. Doesn't matter.

Why did Datsun put that valve there? What is it's function? Why would you want to have that valve open? Ever? Why didn't you just cap off all the lines at the engine and pull the carbon canister off your car?

I'm trying to confirm that you know what the purpose of that purge valve is.

[matt90gt]

Watch the video I posted.  That will show you how the system is supposed to work. Should help explain the purge valve and function - or confirm your knowledge of how the vapor canister works. 

Why didnt I delete the unit?  didnt want the smell of fuel vapors. 

[Captain Obvious]

1 minute ago, matt90gt said:

Watch the video I posted.  That will show you how the system is supposed to work. Should help explain the purge valve and function - or confirm your knowledge of how the vapor canister works. 

Why didnt I delete the unit?  didnt want the smell of fuel vapors. 

That video was done by someone who knows how the system works. I'm not asking if he knows what the purpose of the purge valve is.

I'm asking YOU what the purpose of the purge valve is. What is the purpose and why would you ever want it to open?

[Captain Obvious]

1 minute ago, matt90gt said:

If you want to add to my post with your knowledge for the forum, please do.

I think I did that already. That's what my first three posts were.

My first post was pointing out that your modification does not work.
My second post was telling you how to fix it and actually make it work.
My third post was additional info for you to help you understand why what you have done does not work.