Gas cap

[Carmine]

Hi everyone, I don't remember if I posted this question before or not. If I did, my apologies, and here is my question. I took my '64 SS out today to a local show. I stopped enroute to get gas. The needle said it was approaching empty. I pulled up to the pump and undid the gas cap. I have to tell you, there was alot of pressure behind this cap. It wasn't just a little hissing noise; there was was serious force. I've never experienced anything like it. To my knowledge, the fuel delivery system is stock. I looked at the cap which appears to be new but I can't tell if it's vented or not; or even required?? I really don't think this is proper. Too much force/pressure behind it when I undid it. Could I have been more empty then I thought and this added to the pressure?? I'm thinking about leaving the cap on only half way. The car runs fine considering it was about 90 degrees out. This is not the first time this has happened. Has anyone experienced this before??? Thank you, Carmine.

 

[64ss409]

Mine has never done that. I suspect you have a non vented cap.

 

[62impala409]

The original cap is stamped "VENTED" in big letters. Definitely change the cap. The fuel tank has to be vented to avoid pressure buildup.

 

[Carmine]

Thank you everyone for your replies. I haven't looked at the present cap to see what it says but I will. Took a look at Hubbards Impala Parts and saw that they sell these caps if need be. It's definitely way too much pressure.

 

[Carmine]

Was out in the garage earlier and undid the gas cap. Even sitting overnight, there was still some pressure but not as nearly as much as the day before. Looked at the cap and didn't see anything about it being vented. Didn't give the appearance of it being vented either. Just ordered a new one for this particular year so it should be good.

 

[models916]

Drill a small hole in it till you get a new one.

 

[Carmine]

Well, my new cap arrived today. Can't believe it, it's identical to the old one. No where does it say vented. This cap is specifically for a '64. It will be drill time tomorrow. I'm going to take the old cap, figure out where would be the best spot to drill it, and make it a vented cap. Sometimes this stuff gets old real fast.

 

[Carmine]

I took the old gas cap apart this morning which is identical to the new one. In the center, there is a tan colored what I'll call a diaphragm with 5 small holes in it. The way it's positioned in the cap, if you suck on it, it allows air to come in. If you blow into it, it blocks the air from going out. I understand allowing the air to come in to help the fuel pump pull the fuel. But, I don't understand the opposite. Far from being an engineer, doesn't gas slosh around in the tank as you drive, creating pressure/fumes?? If so, how is this pressure allowed to be expelled?? It can't be released through the cap that I can see. Both caps donot say vented but they do caution you to release them slowly to avoild being sprayed by gas. Hmm................?? Nonetheless, I left the diaphragm off and drilled 3 small holes through the cap. I really wouldn't have been too concerned, but it was alot of back pressure that got my attention. Guess we'll see what happens, Carmine.

 

[Don Jacks]

The allowance of the cap to allow suction could also be there to help eliminate fuel tank collapse.Those tanks in those cars were supposed to be vented,I filled up many of them in my youth.We didnt see non vented caps until the introduction of "emission" controls.

 

[Carmine]

The allowance of the cap to allow suction could also be there to help eliminate fuel tank collapse.Those tanks in those cars were supposed to be vented,I filled up many of them in my youth.We didnt see non vented caps until the introduction of "emission" controls.

I hate to be ignorant here, but if those tanks were suppose to be vented, would this be accomplished by the cap alone or was there something else in place to do this?? If it was by the cap alone, I'm not sure how this could happen. My cap doesn't allow pressure to be expelled. This cap was specifically ordered for my '64 so it's not one size fits all. I'm still somewhat puzzled but I'll see how those drilled holes affect anything. Will report back, Carmine.

 

[mpris]

This is the cap on my 64. It is a Stant.

 

[Carmine]

Thanx for posting that pic. Mine is similar but not identical. Where you have the one hole, I have 5 small ones. It looks like you have a cork gasket and mine is rubber. Hope to go out with the car this week and will check it then.