ChuckD

Guys: I am just not getting it. I have found a great diagram that shows the system in my 1982. My system is intact and I am going through hoses and vac lines at this point. I am considering just removing the charcoal canister altogether but I like to fully understand things and have some piece of mind before I make changes.
1. I think the temp switch controls the small vac line that operates the large vac valve. That large vac valve opens the vac source at the TB to pull air directly from the fuel tank overflow tank, correct? What are the conditions for this to occur?
2. When #1 happens, the fuel vapors bypass the canister altogether, correct?
3. When the vac valve does not allow the vacuum to work (#1), does the air box provide some type of vacuum to allow the tank to vent?
4. Does the charcoal canister work in either direction? The diagram i found shows that air can move both directions through the canister.
5. I don't understand how the A/C valve works. I was told in a post it allows more air to increase idle during AC operation that is unmetered and that must come from the tank vent or the air box line. If I remove the large vac valve, will I still need to provide and air source to the AC valve, like maybe from the air box or just put a small filter on the vac connector that would be left?

I appreciate any help. Thanks!

ChuckD

Nothing?

Ducman82

AC valve is easy. it allows air to bypass the throttle plate, there by raising the idle. DONE.
leave it as is plumbed in etc.

as far as the vapor canister. either leave it and replace all parts, or un hook it.

ammonman

When the purge valve opens it is allowing manifold vacuum to pull vapors from the tank and vapors accumulated in the charcoal canister while the engine was off into the intake to be burned. Flushing air to the charcoal canister is supplied from the air cleaner. Vacuum is applied to the purge valve when the coolant temp is above the temp switch setting (+58C, +136F) and the throttle is partially open. The purge valve vacuum port on the throttle body is on the air cleaner side of the throttle plate. When the throttle plate is closed there is low vacuum at the port. As the throttle opens above idle vacuum is applied to the purge valve. As the throttle opens wider toward wide open, manifold vacuum drops and there isn't enough to keep the purge valve open so it shuts. Fuel vapors only accumulate in the charcoal canister when the engine is off. The tank is always vented through the charcoal canister regardless of engine on or off. When the engine is off the pressure from evaporating fuel in the tank escapes through the charcoal canister and into the air cleaner housing. The charcoal captures the fuel vapor when this flow occurs. When the engine is on air is drawn through the fuel tank and through the charcoal canister via engine vacuum to draw the accumulated fuel vapors in the canister and any new vapor in the fuel tank into the intake manifold to be burned in the engine. The charcoal canister does allow flow in either direction. However, flow is dependent on a difference in pressure. Hence, when manifold vacuum is applied to the canister via the open purge valve flow is from the air cleaner through the canister to the manifold. When the purge valve is shut flow only occurs when fuel vapor pressure in the tank is greater than atmospheric pressure and then flow is from the tank through the canister and then to the air cleaner housing to atmosphere.

With the cost of fuel these days it makes sense to me to retain, capture, and burn every drop I paid for. The charcoal canisters rarely go bad so you are left with a couple of valves, some fuel vapor line, and some vacuum line to replace to ensure the system works as designed. It really is a pretty simple, elegant solution to reducing fuel vapor emissions.

Hope this helps.

Mike

ChuckD

I really appreciate the responses. I have been bugging Ducman via PMs...so thanks for not getting mad at me! I need to have this all explained to me to verify what I was thinking. The explanation of where the vacuum comes from to trigger the purge valve is very helpful.

I think I am going to do the following, after I remove the charcoal canister. (I figure I am doing way less damage to the world than a lot of other folks in the news.) I am thinking I will run a line from the fuel tank vent hard line directly to a T-fitting that will branch over to the purge valve and also up to the air intake. That way, I can leave the system in tact but just bypass the canister. I will smell gas when off, but oh well. The engine will still pull vapors when it is supposed to as well. Also, it will leave the AC valve in place. I must admit I am pretty dense and still don't get that additional circuit. Mike, if you see this, will you weigh in and explain it to me? I assume that when it's activated, it opens the valve and draws air from the vent system through the purge valve. As far as I can tell, this air is pulled pre-throttle plate and added post-throttle plate up in the plenum. What I don't get is that all of this air is post-MAF. I am thinking when the AC valve is activated (and the purge valve must be open to allow any of this to occur, right?), that air is added pre and post throttle plate and that raises the idle. I guess this air that is added directly to the plenum has a much greater affect on idle speed than the air that can be pulled by the base of the TB. Is this correct? if so, why is air added to plenum more effective than air added pre-throttle plate? Thanks for helping me figure this system out. I sincerely appreciate it.

ammonman

Why remove the charcoal canister? Is it damaged or plugged? It has no effect on the overall performance or horsepower of the engine. It only serves to collect evaporating gasoline vapors while the engine is off. When the engine is started and comes up to temp the purge valve allows these collected vapors to be routed to the engine to be burned.

The A/C valve just allows a known, set amount of air to get through the airflow meter and into the intake in addition to the mass air flow that is being controlled by the throttle. In effect, this valve acts like a throttle opening "increaser" by allowing more air to pass through the airflow controlled fuel metering system when the idle needs to be raised to compensate for the additional load of the A/C compressor on the engine. I can't find a diagram of the hose connections for this particular valve to see where it connects into the system. Can you post a photo or diagram of the hose connections?

Mike

ChuckD

I don't have a diagram or photo. This valve hangs from the cross brace and connects via 3/8" vac lines (using 3/8" fuel line because it's closest to correct metric size) to the plenum on one side and the other side connects to a T on the line that runs from the purge valve to the base of the TB. That T connector is right in the area of #2 spark plug. That's assuming cylinders are numbered the same way as on a SBC. Opening the AC valve allows air from the purge valve to split at that T and enter at both the base of the TB (pre-throttle plate) and also at the plenum (in front of plenum).

Regarding the deletion of canister: This car is in bad shape aesthetically and has no value at all to anyone but me. I am going to keep it running and drive it, but it's not worth trying to restore the entire interior. Sunroof leaked and interior was destroyed. Wiring is a mess as well from what I can see. HVAC not working at all. I am planning to simplify it as much as possible and make sure it runs at its best. Deleting things that have no effect on performance. It's cheaper to delete than try to replace the all the lines that are literally crumbling all over the engine bay. The cloth-covered lines are all hard and crumbling, including the one that went from canister to air box. Same goes for the lines from TB to purge valve. Just removing things that can go away here in my state. I really do appreciate your assistance with the information. Thanks!

ammonman

I think (don't have a clear picture of the hose routing) that the flow path when the A/C valve is open is around the throttle plate, not through the fuel vent system even though it sounds like they share plumbing according to your description. Flow will occur along the path of least resistance and be proportional to the greatest difference in pressure. At idle both the A/C valve flow path and the fuel vent flow path have high vacuum at the post-throttle plate connection and atmospheric pressure (or pretty lose to it) at either the fuel tank or the pre-throttle plate connections. However, the resistance to flow is lower through the A/C valve piping (fewer restrictions and shorter path). When the A/C valve opens this path is at a higher pressure difference and lower resistance to flow than through the vent system. Hence, a small amount of MAF-sensed air is allowed past the closed throttle plate during idle to increase idle speed when the A/C is adding to the overall engine load. When the throttle plate opens during driving there is no longer a difference in pressure between the pre- and post-throttle plate ports through the A/C valve so flow through the valve stops. At the same time, there is still a large pressure difference between the purge valve connection to the throttle body and the outside atmosphere through the tank vent system so flow travels this path to purge the canister and fuel tank of vapors (as long as the engine is warm.) Make sense?

Bulk fuel/vapor hose is cheap. I don't think any of those lines are pre-formed. They just appear to be after cooking in the engine bay for 30+ years. Just my $0.02 though.

Mike

dr bob

If you decide to delete the canister and purge valve, you get to invent your own tank ventilation strategy. Find a check valve that's fuel-proof or you'll have a garage full of gas fumes on a warm day.

The cloth-covered vapor hose is a commodity item at import parts stores. It's been used on VW cars since the beginning of recorded history. The operation of the valve is pretty simple, from ported vacuum on the throttle body. The hose from the AC valve is there to supply filtered air to that loop, so opening the upstream side feeds purge air from the canister to wherever it needs to go. OK to put it in upstream of the throttle or through the AC loop to downstream. Since it's fuel vapor, no issues with adding it past the arflow sensor. The system is amazingly simple and effective, easier to keep than to bypass or vent safely.

ChuckD

I think that the purge valve will stay in place. I don't see how to keep the AC valve in place without keeping the purge valve. I think I will run the flexible vent line up to a T. One branch will go over to the purge valve. The other branch will go up to the hole in air box. I think I will have to step up the size a bit to fit the hole in air box. This should maintain the functionality of the purge valve and still allow the tank to inhale a bit when fuel levels drop. I guess a check valve would also be a good idea. Are those something that need to be oriented up and down in order to work? Would using that defeat the ability for tank to take in air? I keep reading that gas caps are sealed and you have to be able to let the tank vent.

Ducman82

that AC valve has NOTHING to do with the purge set up. leave it be, dont change anything with it. it is nice and happy where it is at and how it works. :-P

Ducman82

i vented my take behind the rear wheel with no gas smell when hot out. it can be done.

ChuckD

DucMan: The AC valve does connect to the large vac line from the base of the TB that goes to the purge valve. There is a T just above the spark plug (#2 on a SBC) where one side of the AC valve connects in. The other side of the AC valve connects to the front of the plenum. I don't see how I can modify anything with no regard to this circuit. It needs to have access to unmetered air via where it Ts into the purge line line. This is why I haven't responded to your e-mail...this valve is part of the purge system. If I am still not getting it...slap me. I must need it.

Kazoocruiser

I know this is a really late response to what you posted here, but it answers both questions I had while trying to sort out evaperative emission related issues on a 1981 Chevrolet C10 with an inline 250 6 cylinder. I have a carbon canister, but the engine had a four-port thermal switch that was broken and not hooked up. I figured out it is a 2 stage switch, with the first port opening at 140 degrees, and the second at 160, which I figured out was to open the purge valve. What i didn't know was what kind of vacuum to route to it, and your response indicates ported vacuum is correct for the application.

Thanks for the explanation. The Internet to the rescue.