recently I've been having issues with the vented fuel cap on my 50 chevy After driving about 20 minutes Was hearing a thump. I finally diagnosed the fuel tank behind the seat was sucking in. When I get to my destination I would release the cap and hear all the air rush in. I tried another cap thinking the first new one was defective, but same result. Anyone ruiing into this issue? Any remedies? On another forum someone suggested removing the spring under the vent. Sounds like these new caps are not letting enough air in.
Both of my cars do the same thing when releasing the vented gas cap. The Hard Top has a 235 while the convertible has a 350. They both have new tanks and caps. As long as your not stalling I would say not to worry about it.
i had a problem with a cap made for a "modern" car. the neck size plugged the outer edge venting capability. here is one from chev-of-the-forties. https://www.chevsofthe40s.com/detai...p_NonLocking_Polished_Stainless_2852_Car.html
I had a similar problem with a 48 chevy car with an original tank. Really hot weather made it much worse as pressure built up in the tank. Actually had gas spray out about 15' a couple of times when I stopped to refuel. This didn't start until I left my original stock cap at a gas station. I tried 3 or 4 different new vented caps. Nothing seemed to work. Finally took the gasket out of the cap and it cured the problem.
Just get a non vented cap and drill a hole in it, that is what I used to do on my bikes. The vented Harley caps were notorious for spewing fuel all over your tank so I would get a non vented cap from the non vented side of a dresser and drill a 1/16 hole in it. Worked like a champ.
Yeah, I'm pretty sure the valve with spring is your problem. I don't know anything about Chevys, but old cars in general had no valving in there...the vent just connected with air, sometimes right out the top, other times sideways into the area covered by outer rim of cap. Drill a simple cap, get an old one, or simply shove a piece of wire up into the modern vented one so the valve is propped open and your troubles will vanish.
I'm curious why upon hearing your tank "go thump" that you would even consider not solving the problem It is a problem and needs to be addressed. Easy fix...plenty of advice above!
Yep Gene you are absolutely correct and a lot of it is bone head simple, if I can do it then it must be something pretty easy to do.
Thank you for the link, I appriciate it hearing the thump my initial thought was a loose under bed spare carrier, then I was thinking loose lug nut. Very odd to find it comming from the fuel tank. Thanks for the above suggestions, Ill try one of them and report back. I have used this brand cap in the past and not had this issue. Maybe they went to tighter springs for some reason. Usually the rubber rots out so I change the cap when I see it crack.
I see a lot of bs up above but the real problem is that your "vented" cap is not venting. It has to vent both ways and what is happening is that when the fuel pump pulls gas from the tank there is no air going in thought the vented cap to replace the gas. Just the opposite happens when the rig sits in the hot sun. the cap doesn't vent and pressure builds up in the tank. I went through three new off the shelf Stant brand "vented" caps before I got one that worked on my 51 Merc 30 years ago. Drive a few miles and the engine quit. The first time I walked a mile to a gas station and got gas and walked back, put the gas in the car and fired it right up. The second time I had a can of gas in the trunk and when I opened the cap I heard the sides of the tank pop back out. Probably the easiest fix is drill the cap so it vents all the time or get an old original cap that vents. At any rate the tank has to have some sort of venting system in place. Be that a vented cap or a hose running to a charcoal canister like a lot of ot cars have or vent hose running up high enough and in a spot it won't burp gas or gas fumes inside the car.
My '40 Ford pickup uses the stock tank location under the rear of the bed with a short, fairly horizontal filler neck. All stock. It always takes forever to fill the tank or it will burp back up all over the fender. In addition, it leaks gas out of the vented cap when cornering if the tank is more than 1/2 full. I got rid of the old cap and replaced it with a new vented cap and it still leaks a bit but less. I would put a proper separate vent on the tank but there is no vertical height to elevate it high enough that it wouldn't dribble as well. It is a pretty poor venting system I think and I wonder what the original trucks did.
When I have a long filler neck ( like to a gas door or the like) I braze a nipple on the top end if the filler and then run a hose down and under the vehicle with a filter on the end of it. You can find cheap fuel filers that are not directional if you look a bit.
I drilled a hole in the top portion of the filler neck, above the rubber hose connector. Then I inserted a brass nipple from the hardware store. Ran a rubber vacuum line to points beyond and fixed it that way. No gas smell, no leak, no problem.
Where in the cap are you guys drilling this hole I found a nos cap but dont like the fit, may have been mislabeled and not for my bolt I think ill try to modify the new one that is not venting properly You can see the nos in he background and it has the hole My guess is anywhere behind the plastic vent, just inside the gasket Just want to make sure Dont want any chance of a self siphon Thanks
I am not sure that a '50 Chevy cap had the little plastic deal in it but not to worry. Put the can together and poke a drill bit all the way though everything, it will vet for sure after that, using the existing hole. Note: if I had a different cap and had not tried it yet I would try it first. If it works leave it alone.
The plastic vent is on the modern aftermarket The nos is in the background with the keys I have a couple of these modern caps. Ill try drilling one Maybe pull the spring out the other Looks like the white needle is the check valve I just dont see a path to outside air for the make up? These use to work just fine, except the rubber would rot out so I have to change the cap every couple of years. It was just recently that the last new ones would not vent
the path the air takes is above [when installed] the rubber/steel outer gasket. the ones i tried to use on older cars would get squeezed shut because they were not the right size/depth for the fuel neck. trying to drill the inner metal cap without harming the out side chrome will be hard. maybe use a die grinder to grind through it.
well finally got back to this issue under the valve are 4 holes already so there was no need to drill. You can see the 4 holes in the pic above two springs in the valve assembly I first removed the large spring. That helped with the tank building pressure but not with the make up air. On a drive to work the tank would still thump from being sucked in Then removed both springs on the valve assembly. this seems to let it vent both ways. thanks
I haven't paid much attention to whether the gas cap huffs or not when I open it on my newer vehicles. I usually don't open the cap unless I'm low either. I know I've had cars with the charcoal canister. It seems to me that EPA non vented caps would have to vent somewhere as pressurized gasoline could be pretty dangerous in an accident. Or are they risking our lives in the name of some minute "air pollution".
The small spring attached to the white valve needle was the one preventing the makeup air. Ill run without both springs. Ill report back if I have any issues with splash out, if I do Ill try cutting a few coils off the needle spring. Not sure why the cap has such strong springs. Seems like the low pressure/vac of the mechanical fuel pumps cannot overcome the spring pressure
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