Should the gas cap be vented or non vented?

[VadersV6]

My old gas cap, which was an autozone part..got left at a gas station and lost, so Ive been using another gas cap I had that I bought when the guys at Kragen didnt know if my cap should be vented or not. Their parts book showed both part numbers for the same car. The cap thats on there now is a locking type and says "turn counterclockwise and wait for hissing to stop, then continue turning" or something like that. I assume this would be a non vented cap? My tank has hissed 1 time in the past 50,000 miles. Dont know why but it was just that once. And going back and forth betwen different gas caps made no difference.
So Im just curious as to which one is correct? The EVAP system is still in place and functional as far as I can tell.

 

[BlackBandit]

If I had to guess I'd say it should be non vented so that the tank will vent through the charcoal canister.

 

[VadersV6]

But if its not vented then there is no movement of air/gas fumes. The engine will be sucking on it but it cant move given there is no vent.

 

[TurboDave]

It vents to the charcoal canister.

You don't want it venting to the atmosphere through the cap, especially there in Kalifornia.
The smog nazi's would love you for that!

 

[VadersV6]

The charcoal canister vacuum creates the "draw" on the tank, but without a vent, the tank will be in constant vacuum, which is not good for the pump and the gas..it can get really hot. Is there another vent somewhere?

 

[kjhansen]

In Texas, on "modern" fuel injected cars like the Buick, during vehicle inspections, they run a test on the gas cap to make sure it seals. It's not supposed to let air into the tank. With an ELECTRIC fuel pump (i.e., an electric motor--potentially making sparks as brushes rub against spinning contacts on the armature) IN the gas tank, you don't want a lot of air getting down in there. In any case, the engine doesn't suck on the gas. The pump pushes it and it vents through the canister, as TurboDave noted, then the unused gas recycles back to the tank, still under pressure. It's a closed, constant flow, pressurized system--not a vacuum system. Fuel is going around in a big circle all the time. Use a non-vented cap. One thing I learned working for a parts store is that the lists in the computers are often so generic as to be worthless. Kragen's probably lists the naturally aspirated, carbed engine with the external vacuum-operated mechanical fuel pump (which has to have a vented cap) and the fuel-injected, electric-fuel-pump-in-the-tank engine as the "same car." And these parts stores hire people with no mechanical knowledge whatsoever, so they depend entirely on what the computer tells them. Not good.
Keith

 

[VadersV6]

Ive got the check valve going to the charcoal canister so it provides constant vacuum but is closed under boost. This is correct isnt it?

 

[TurboDave]

Ive got the check valve going to the charcoal canister so it provides constant vacuum but is closed under boost. This is correct isnt it?

It's not really under "constant" vacuum. It's only vented when commanded to do so through the solenoid mounted on the canister.

 

[Gary Wells]

Should be non-vented, locking gas caps generally leak to the atmosphere whether they are built as a vented or a non-vented, & Kali smog will bust you for a leaking and / or vented one. but generally give you a chance to exchange and or borrow one before writing you up.

 

[Junior Samples]

Ah ha I knew it

I had posted Q on my canister solenoid and received alot of post's telling me to ditch it and vent but somewhere in the back of my teeny little brain I knew it was a closed system.

I stated that me only being used to carbed app's with mech fuel pumps, i knew system had to be vented but wasnt sure on intank pump with sfi.

kinda thought that being a closed system, as pressure built up, solenoid would open and pull some fumes in thus relieving pressure.

So today after work I yanked my canister, it was bad, solenoid is bad, and now hmmmm back to square one..should i do what i refer to as the BTO treatment and Let it Ride?

Note: BTO bachman turner overdrive

hhhmmm decisions decisions

 

[Junior Samples]

Just got in from garage, car idle'd alot better with canister unhooked but of course it was plugged with bad solenoid stuck open and packed full of charcol pebbles along with the line so the tank was'nt releasing pressure properly.

But still eratic idle although I've got 16 inches steady, I know its in my injectors and they are next to go.

Im still geared up on this car although both vettes are turn key all day long but it gives me another reason to go to garage so i dont have to hear the woman B*&$%