testing gas tank ventilation

I've a 65 dart with a /6. How do I test if the tank is properly vented? The gas cap is an after market one with a key and not vented but I couldn't find anything in the manual to say that it needs to be. I also noticed that there is a line on the fill tube for venting anyway. So how do I test if it's working properly? and not plugged. Just to be certain. Thanks.

why hasn't any one replied to this post?

I also have a 65 dart w an aftermarket locking gas cap. I have on occaison heard a whoosh when I take the cap off & the only time I run into a gas starvation problem is when I inadvertently let the gas gauge needle get down to the left side of the E so I am assuming that I am OK. Pull the vent line off off the filler neck inside the trunk at the short neprene connector & blow thru it.

I blew through the vent line at the filler hose. Air went threw but it was like trying to blow up a new balloon. Should there be that much resistance?

the line is vented directly to atmosphere & I imagine rust has blocked it to a certain extent but you are OK there.

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I also have a 65 dart w an aftermarket locking gas cap. I have on occaison heard a whoosh when I take the cap off

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is the whooosh sound making vacuum or releasing pressure ?