Jim Saraceno Posted February 19, 2008 Report Posted February 19, 2008 I recently bought a new gas cap at NAPA for my '35. When I went to fill the tank I heard a small "whoosh" sound as I pulled the cap off. Apparently, the new cap creates a vacuum as the gas is used up in the tank. I'm sure the old cap was not air tight. Does anyone know if there is some pressure rated check valve that will let air in at a certain point? Were the old gas caps open to outside air? I re-installed the old one because I wasn't sure if this could create problems as the vacuum got greater. Quote
Don Coatney Posted February 19, 2008 Report Posted February 19, 2008 Jim; The gas cap must be vented. If not your engine will quit running as soon as the vacuum in the tank becomes equal to the suction capacity of the fuel pump. Quote
captden29 Posted February 19, 2008 Report Posted February 19, 2008 on my 54 windsor there is a vent line between the cap and tank, easily visible if you have one. the rubber hose on the vent line deteriorated and clogged, it was then doing the same whoosh you describe and actually starting to crush the tank. once i replaced that small vent line all was back to normal. it is a small hose, not much bigger than a distributor vacuum hose. dennis Quote
Normspeed Posted February 19, 2008 Report Posted February 19, 2008 Depends on the year and model. If you don't have the little vent tube, you should probably have a vented cap. I had a non vented on my 53 wagon and it was incorrect but only for the wagons that year. I did a little modifying so now it's vented. No more whoosh. Quote
captden29 Posted February 19, 2008 Report Posted February 19, 2008 on my 54 windsor there is a vent line between the cap and tank, easily visible if you have one. the rubber hose on the vent line deteriorated and clogged, it was then doing the same whoosh you describe and actually starting to crush the tank. once i replaced that small vent line all was back to normal. it is a small hose, not much bigger than a distributor vacuum hose. dennis Quote
Oldguy48 Posted February 19, 2008 Report Posted February 19, 2008 Not sure about the 35s, but I recall an incident with a 69 Dodge van, when my father purchased a new gas cap for it. The new cap was not vented, and the gas tank actually collapsed due to the vacuum created as the gas was consumed. We decided maybe we could pop it back to normal with compressed air down the filler neck! Bad decision! The tank did kinda pop back to normal, but when I tried to relieve the pressure in the tank (I had a rag wrapped around the air hose & stuck in the filler neck), I got a face full of gas right square in the eyes. Spent the next hour or two at the local hospital emergency room. Hopefully someone else can learn from my dumb mistake!! Quote
Norm's Coupe Posted February 19, 2008 Report Posted February 19, 2008 My guess would be the gas cap is supposed to be a vented one too. I know the 30's Fords all use the same gas cap as the 48 Plymouth and they are vented. Quote
Jim Saraceno Posted February 19, 2008 Author Report Posted February 19, 2008 Looking at the gas cap I just realized there is a check valve in the center. I blew and sucked on it and it opens to let air in, but not out. So I guess that answers my own question. It must let air in. The tank itself does not have any vent tube. I guess I just need to make sure the check valve is open to outside air as opposed to where a vent tube would be. So... just where does your vent tube go? Quote
Don Coatney Posted February 19, 2008 Report Posted February 19, 2008 Looking at the gas cap I just realized there is a check valve in the center. I blew and sucked on it and it opens to let air in, but not out. So I guess that answers my own question. It must let air in. The tank itself does not have any vent tube. I guess I just need to make sure the check valve is open to outside air as opposed to where a vent tube would be. So... just where does your vent tube go? So was the wiff of air going into the tank or coming out? Gasoline will expand with heat and build pressure. This could cause carburetor flooding if the tank pressure is greater than the boyancy of the carburetor float. I would think a gas cap vented in both directions would be the way to go. Quote
55 Fargo Posted February 19, 2008 Report Posted February 19, 2008 Jim, on this tank you need a vented cap, this allows the air to intake and exhaust, you do not have a vent valve on the tank if it is OEM. The suction, will collapse this tank iof given enough time, when the gas is hot it will also expand, it needs to vent out, as well as be able to draw air in if needed to prevent a vaccuum effect from the fuel pump....... Quote
Jim Saraceno Posted February 19, 2008 Author Report Posted February 19, 2008 So was the wiff of air going into the tank or coming out?Good question... I assumed the air was going in but maybe it was going out. Jim, on this tank you need a vented cap, this allows the air to intake and exhaust, you do not have a vent valve on the tank if it is OEM. So I guess I need to take that check valve out of there so the air will flow in both directions? Quote
central52 Posted February 20, 2008 Report Posted February 20, 2008 How can you tell if a gas cap is vented. I have the original one on my 47 Plymouth, and I don't see any holes anywhere. I bought a new one from Andy Bernbaum, and it doesn't have any holes anywhere. Where does the air go in and out? Ed P. Quote
55 Fargo Posted February 20, 2008 Report Posted February 20, 2008 It is made in such a fashion that it does not seal air tight against the filler neck pipe, that way it can draw or expel air.............Fred Quote
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