Evap Canister
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
Evap Canister
I am re-assembling a stock 1975 L-48 four speed Vert. I acquired it in pieces.
Lars informed me the carb is not stock, however it is running fine, so I have no plans to change the carb.
My question is related to the Evap canister. I re-installed the canister and now realize I have no point to connect a vacuum source.
Will the canister simply vent on its own or must I have a vacuum to draw the fuel tank fumes through.
I don’t want to cause my gas tank to become pressurized.
I could just leave the tank vent open or maybe I tee into another vacuum source.
Any suggestions?
Lars informed me the carb is not stock, however it is running fine, so I have no plans to change the carb.
My question is related to the Evap canister. I re-installed the canister and now realize I have no point to connect a vacuum source.
Will the canister simply vent on its own or must I have a vacuum to draw the fuel tank fumes through.
I don’t want to cause my gas tank to become pressurized.
I could just leave the tank vent open or maybe I tee into another vacuum source.
Any suggestions?
Pro
Joined: Nov 2014
Posts: 728
Likes: 346
From: Smyrna, TN
I would block off the vapor line at the vapor separator valve mounted to the tank.
But you will have to get a vented gas cap.
The canister has to have a vacuum to work, but it is not necessary.
You will also need to block or vent the port in the valve cover.
But you will have to get a vented gas cap.
The canister has to have a vacuum to work, but it is not necessary.
You will also need to block or vent the port in the valve cover.
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
Burning Brakes
Joined: Oct 2002
Posts: 1,040
Likes: 109
Not sure why you would want it not working.
You need a vacuum source to open the vent valve to purge the canister thru the PCV valve into the engine
Is car parked in an enclosed space.
I installed a new tank, vacuum to atmosphere, & connected to the original metal vapor line running to the canister, plumbed in a new vent valve so it
will operate as GM intended.
Your car = do what's right for you.
You need a vacuum source to open the vent valve to purge the canister thru the PCV valve into the engine
Is car parked in an enclosed space.
I installed a new tank, vacuum to atmosphere, & connected to the original metal vapor line running to the canister, plumbed in a new vent valve so it
will operate as GM intended.
Your car = do what's right for you.
Burning Brakes
Joined: Oct 2002
Posts: 1,040
Likes: 109
garage might smell depending on temperatures.
Again take all the answers throw them in your brain, sort = do what works for you
Pro
Joined: Nov 2014
Posts: 728
Likes: 346
From: Smyrna, TN
I just assumed (perhaps wrongly) that the OP was going that direction with a different carb and was only concerned about venting his tank...
Le Mans Master
Joined: Sep 2002
Posts: 5,236
Likes: 898
From: Myrtle Beach SC
Your 75 was the first year for the smaller gas cap...finding a vented cap may be a challenge. As others note, fuel vapors have to go somewhere, when you've been around these later C3s, it is obvious when the cannister isn't working and vapors fill the air. Keeping such a configuration in a garage...isn't safe! 
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Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
Thank you all for your responses.
I have always tried to keep my Corvettes (my fourth) as close to original as feasibly and financially possible. I do not have all the original parts to this car and am on a very very limited budget, as is much of the nation right now. If I can simply T into an existing line and solve my problem, great.
I do not park this car in an enclosed garage - it is covered under an open carport. However, that could change in the future, so I will try to keep the tank vented as originally designed.
If I T into the PCV line, will there be enough vacuum to open the PCV valve and purge the canister?
I have always tried to keep my Corvettes (my fourth) as close to original as feasibly and financially possible. I do not have all the original parts to this car and am on a very very limited budget, as is much of the nation right now. If I can simply T into an existing line and solve my problem, great.
I do not park this car in an enclosed garage - it is covered under an open carport. However, that could change in the future, so I will try to keep the tank vented as originally designed.
If I T into the PCV line, will there be enough vacuum to open the PCV valve and purge the canister?
Safety Car
Joined: Oct 2007
Posts: 4,380
Likes: 1,742
From: Seattle Area Washington
It is my understanding the amount of vacuum needed to open the purge valve on the canister is minimal.
Your PCV valve should work just fine.
It is also important that the fuel vapor line (from sending unit to canister) be connected.
This vapor line is "T" to a port on-top of the tank.
This port is what's called the equalizer port. The purpose is to keep pressure from building up between the tank and the bladder that could collapse the bladder, that is if it's still there...
.
Your PCV valve should work just fine.
It is also important that the fuel vapor line (from sending unit to canister) be connected.
This vapor line is "T" to a port on-top of the tank.
This port is what's called the equalizer port. The purpose is to keep pressure from building up between the tank and the bladder that could collapse the bladder, that is if it's still there...
.
Pro
Joined: Nov 2014
Posts: 728
Likes: 346
From: Smyrna, TN
Your 75 was the first year for the smaller gas cap...finding a vented cap may be a challenge. As others note, fuel vapors have to go somewhere, when you've been around these later C3s, it is obvious when the cannister isn't working and vapors fill the air. Keeping such a configuration in a garage...isn't safe! There are a whole bunch of cars built before 1970 sitting in garages with vented gas caps and no EVAP system.
Race Director
Joined: May 2006
Posts: 18,733
Likes: 2,582
Thank you all for your responses.
I have always tried to keep my Corvettes (my fourth) as close to original as feasibly and financially possible. I do not have all the original parts to this car and am on a very very limited budget, as is much of the nation right now. If I can simply T into an existing line and solve my problem, great.
I do not park this car in an enclosed garage - it is covered under an open carport. However, that could change in the future, so I will try to keep the tank vented as originally designed.
If I T into the PCV line, will there be enough vacuum to open the PCV valve and purge the canister?
I have always tried to keep my Corvettes (my fourth) as close to original as feasibly and financially possible. I do not have all the original parts to this car and am on a very very limited budget, as is much of the nation right now. If I can simply T into an existing line and solve my problem, great.
I do not park this car in an enclosed garage - it is covered under an open carport. However, that could change in the future, so I will try to keep the tank vented as originally designed.
If I T into the PCV line, will there be enough vacuum to open the PCV valve and purge the canister?
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
It is my understanding the amount of vacuum needed to open the purge valve on the canister is minimal.
Your PCV valve should work just fine.
It is also important that the fuel vapor line (from sending unit to canister) be connected.
This vapor line is "T" to a port on-top of the tank.
This port is what's called the equalizer port. The purpose is to keep pressure from building up between the tank and the bladder that could collapse the bladder, that is if it's still there...
.
Your PCV valve should work just fine.
It is also important that the fuel vapor line (from sending unit to canister) be connected.
This vapor line is "T" to a port on-top of the tank.
This port is what's called the equalizer port. The purpose is to keep pressure from building up between the tank and the bladder that could collapse the bladder, that is if it's still there...
.
Instructor
Joined: Feb 2011
Posts: 186
Likes: 63
From: a little West of Houston Texas
No where near the factory configuration, BUT I think it will work, sure looks like a simpler way to do it..
Which brings up a question of why the factory set up has a vacuum controlled valve that has to have vacuum applied before
vapors are removed from the evap canister.
Which brings up a question of why the factory set up has a vacuum controlled valve that has to have vacuum applied before
vapors are removed from the evap canister.
Race Director
Joined: May 2006
Posts: 18,733
Likes: 2,582
No where near the factory configuration, BUT I think it will work, sure looks like a simpler way to do it..
Which brings up a question of why the factory set up has a vacuum controlled valve that has to have vacuum applied before
vapors are removed from the evap canister.
Which brings up a question of why the factory set up has a vacuum controlled valve that has to have vacuum applied before
vapors are removed from the evap canister.
Melting Slicks
Joined: Jul 2019
Posts: 2,294
Likes: 2,016
From: MA/NH Seacoast
Your 75 was the first year for the smaller gas cap...finding a vented cap may be a challenge. As others note, fuel vapors have to go somewhere, when you've been around these later C3s, it is obvious when the cannister isn't working and vapors fill the air. Keeping such a configuration in a garage...isn't safe!
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
First run with this configuration of vent for the Evap via PCV.
First, the raw fuel smell is gone.
A cold engine cranked right up, idled in the 1500 range until the choke settled. Engine then ran smooth and idled great.
Cost = about $1 or less for the T.
Time = about 15 mins.
First, the raw fuel smell is gone.
A cold engine cranked right up, idled in the 1500 range until the choke settled. Engine then ran smooth and idled great.
Cost = about $1 or less for the T.
Time = about 15 mins.
Instructor
Joined: Feb 2011
Posts: 186
Likes: 63
From: a little West of Houston Texas
No I meant the small plastic valve (purge valve) located on the canister on drivers inner fender, I don't think it's temperature controlled/related at all, just a port in the carb, no other devices
the way he has it (T) into the PCV would pull all the time, which I guess would work, vapors build in tank, escape to charcoal canister until start up, purge gets vacuum = opens, pulls vapors into engine
Race Director
Joined: May 2006
Posts: 18,733
Likes: 2,582
thermostatic meaning heat
No I meant the small plastic valve (purge valve) located on the canister on drivers inner fender, I don't think it's temperature controlled/related at all, just a port in the carb, no other devices
the way he has it (T) into the PCV would pull all the time, which I guess would work, vapors build in tank, escape to charcoal canister until start up, purge gets vacuum = opens, pulls vapors into engine
No I meant the small plastic valve (purge valve) located on the canister on drivers inner fender, I don't think it's temperature controlled/related at all, just a port in the carb, no other devices
the way he has it (T) into the PCV would pull all the time, which I guess would work, vapors build in tank, escape to charcoal canister until start up, purge gets vacuum = opens, pulls vapors into engine