Stock gas tank and vents, do I need vapor/fuel overflow?

[billh1289]

I bought mushroom caps from Fox: https://foxresto.com/
You can sometimes find them on eBay too.

 

[BPerry]

Chiming in on this. 73' two line tank vent, efi with the return on the filler vent "haven't had any issues yet".

Currently I have no Charcoal Canister "removed and plugged by the PO".
Condensing tank is present and hooked up.

Yesterday was the first day I really took it out, about 90 miles or so..

What I noticed was a lot of back pressure when opening the gas cap...

Thoughts on all the above? Replace Charcoal canister, vented gas cap?

Bp

 

[DirtDonk]

To make the cap "vented" you have to ventilate it yourself with a drill or a .22 cal. Just kidding, a very small drill does the job. A 22 is too big!
The reason is that they re-designed the caps (different reach on the locking tangs) on purpose so people with EVAP systems in their rigs could not defeat the function of their charcoal canister systems by installing earlier design fully vented caps.
Unless someone has found a retrofittable cap then, your only choices are to either modify an existing one or keep the vapor recovery system intact.

Is your second vent port running up to the condensing tank? You said it's still in the system, so I'm assuming yes on at least one of the hoses.
If that's the case, then in theory you have at least half of the charcoal canister system and should at least still have venting through it. Unless the tank in the cab has it's unused hoses blocked off that is? If so then yes some pressure will build up.

And when you say pressure, was it positive pressure pushing outward when you opened the cap, or negative pressure (vacuum) pulling in? Sounds like positive pushing out but I wanted to make sure.
If so, then it sounds like your one remaining vent is not fully open. Maybe blocked off at the tank?
Got a diagram of how yours is plumbed?

paul

 

[BPerry]

To make the cap "vented" you have to ventilate it yourself with a drill or a .22 cal. Just kidding, a very small drill does the job. A 22 is too big!
The reason is that they re-designed the caps (different reach on the locking tangs) on purpose so people with EVAP systems in their rigs could not defeat the function of their charcoal canister systems by installing earlier design fully vented caps.
Unless someone has found a retrofittable cap then, your only choices are to either modify an existing one or keep the vapor recovery system intact.

Is your second vent port running up to the condensing tank? You said it's still in the system, so I'm assuming yes on at least one of the hoses.
If that's the case, then in theory you have at least half of the charcoal canister system and should at least still have venting through it. Unless the tank in the cab has it's unused hoses blocked off that is? If so then yes some pressure will build up.

And when you say pressure, was it positive pressure pushing outward when you opened the cap, or negative pressure (vacuum) pulling in? Sounds like positive pushing out but I wanted to make sure.
If so, then it sounds like your one remaining vent is not fully open. Maybe blocked off at the tank?
Got a diagram of how yours is plumbed?

paul

Well, im a pretty good shot.... lol

Both vent lines are running to the condensor, new WH tank so they theoretically are not blocked...

The Charcoal can on the passenger frame rail has been removed and the hose is plugged..

Yes, positive pressure. With that being said, if it's a closed system; then the plugged line where the Charcoal canister would have been would create tank pressure. I assume...

Bp

 

[BPerry]

To make the cap "vented" you have to ventilate it yourself with a drill or a .22 cal. Just kidding, a very small drill does the job. A 22 is too big!
The reason is that they re-designed the caps (different reach on the locking tangs) on purpose so people with EVAP systems in their rigs could not defeat the function of their charcoal canister systems by installing earlier design fully vented caps.
Unless someone has found a retrofittable cap then, your only choices are to either modify an existing one or keep the vapor recovery system intact.

Is your second vent port running up to the condensing tank? You said it's still in the system, so I'm assuming yes on at least one of the hoses.
If that's the case, then in theory you have at least half of the charcoal canister system and should at least still have venting through it. Unless the tank in the cab has it's unused hoses blocked off that is? If so then yes some pressure will build up.

And when you say pressure, was it positive pressure pushing outward when you opened the cap, or negative pressure (vacuum) pulling in? Sounds like positive pushing out but I wanted to make sure.
If so, then it sounds like your one remaining vent is not fully open. Maybe blocked off at the tank?
Got a diagram of how yours is plumbed?

paul

Now I'm second guessing myself, gonna take a short trip and confirm the pressure...

 

[DirtDonk]

Yes, it's very likely. You could test that theory by un-capping it.
You might smell gas (which isn't a good thing, but it's good proof!) but it would tell you if it's enough to stop the pressure buildup.

One of the hoses is a vent and one is a return for liquid that condenses to go back to the gas tank, but the system has to ultimately be open somewhere for the pressure to release. And that would normally be the charcoal canister.

This is a perfect time to update the system to the more modern plastic tank as used on the '76 and later Broncos mounted up high on the firewall. Once that is installed then any time you decide to get rid of the condensing tank in the cabin you can do that. Whether it's due to a leak (which happens fairly often), or you want to repurpose that part of the interior for something else, or you just want to get rid of any gasoline inside the cabin, you can then take it out of the equation and run the hoses directly to the high-mounted charcoal canister.
But if your tank is not leaking at this point and you don't want to do the extra plumbing, just connect the new canister and you're done.

You've probably seen some of the discussions on how to do it. The good news is that the later style is still available brand new or can be gotten from the junkyard. Or if you have the old one and only need to bolt it on and go, that's still a viable alternative. Lots of '70-'75 Broncos still running around with their original canisters in working order.

paul

 

[BPerry]

Yes, it's very likely. You could test that theory by un-capping it.
You might smell gas (which isn't a good thing, but it's good proof!) but it would tell you if it's enough to stop the pressure buildup.

One of the hoses is a vent and one is a return for liquid that condenses to go back to the gas tank, but the system has to ultimately be open somewhere for the pressure to release. And that would normally be the charcoal canister.

This is a perfect time to update the system to the more modern plastic tank as used on the '76 and later Broncos mounted up high on the firewall. Once that is installed then any time you decide to get rid of the condensing tank in the cabin you can do that. Whether it's due to a leak (which happens fairly often), or you want to repurpose that part of the interior for something else, or you just want to get rid of any gasoline inside the cabin, you can then take it out of the equation and run the hoses directly to the high-mounted charcoal canister.
But if your tank is not leaking at this point and you don't want to do the extra plumbing, just connect the new canister and you're done.

You've probably seen some of the discussions on how to do it. The good news is that the later style is still available brand new or can be gotten from the junkyard. Or if you have the old one and only need to bolt it on and go, that's still a viable alternative. Lots of '70-'75 Broncos still running around with their original canisters in working order.

paul

So, fumes aren't being pushed out; with the cap removed while running. That I can tell.

I get a lot of gurgling when the cap is removed... so now i'm thinking vacuum...

Stupid question - "One" or "two" port new charcoal canister.

 

[DirtDonk]

Most of the canisters that mimic the stock one have a total of four ports. Two large ones, one medium, and one small.
The small one is for the single line in to the canister from the tank(s).
The medium one is for a carburetor bowl vent.
One large one goes to the air cleaner and the other is simply vented to atmosphere. The vented one gets a vented cap to keep debris out.

Depending upon how you set it up, you can also use the medium one as your outlet to the air filter. I’ve seen that done many times and it seems to work.
Even the factory used it on later model computer control vehicles where the computer controlled a solenoid to open and close.
In those cases both of the large ports are capped with the vented caps.

 

[DirtDonk]

Remember to that no fumes while the engine is running doesn’t necessarily mean that the tank is not pressurizing.
A pressurizing tank is usually the result of starting off the morning cool, then warming up during the day.
Not always something to do with the engine running or the fuel system misbehaving.