Fuel tank pressure

I installed a (new) vented cap but am still experiencing pressure buildup in my fuel tank....normal?

yep

Is it REALLY a vented cap,,,,,,,,is the tailpipe close to the tank?

I get pressure with a new vented cap, gets higher the warmer it is outside.  To test, clean it first and suck on it, should let air in and not out when you blow on it.

donivan65:
Yes (brand new) vented cap...and yes the tailpipe runs right by the tank...wasn't this the factory position (of the tailpipe)?

The tailpipe heats up the tank,,,,,raises the pressure,,,,,,drill a small hole in that cap,,,,,,,,seems to me,,,its hard to find a vented cap,,,,,,,and one that actually works,,,,,,,a vented cap needs to let air out,,,,,,,,the newer cars have non vented caps for the Emissions,,,,,,the tank vents to the charcoal cannister ,,,,,,

,,,what gas cap did you get????

That appears to be the cap I bought...and that was my initial thought (drilling a hole)...thanks!

...and the original tail pipes turned out to like under the gas cap,,,,,not go out to the rear bumper,,,,,feel the gas tank after a drive,,,see if the tailpipe makes the tank hot.......if so,,,,you could wrap it or put a shield on it,,,,,,,,, we have had to drill holes in those new ALLEGEDLY vented caps......

I got a Stant vented cap at AutoZone, the one with the two finger tabs and fits a GM neck, seals tight, no slosh out on curves with a full tank and is vented.  The one with the tabs makes it easy to open and close tight, doesn't look as good as the smooth chrome dome ones, but works.  Its silver plated, could be primed and painted.

looks like this one and says vented and I tested it, you can suck air in but not blow out!

Vantasia: That was the cap I had initially...and it probably worked fine...because the new one did the exact same thing.

donivan65: My pipe does run along side the tank (about 2" at the closest point!) Looking at it closely if I were to angle it about 45 degrees right after the shock it would come out where you said...easy fix...and a small hole in the cap just to be sure!

,,,a vented cap means you should be able to blow air out.......thats its job,,,,,,release the pressure in the tank,,,,,so you dont get your face blown off when you remove the cap,,,,,,,,,,but the hole is really small,,,I doubt that you can even feel air coming out of it,,,,,,,if you get pressure coming out of the tank as you remove the cap,,,either it aint working or it is not a vented cap,,,,,,if there is a warning to remove SLOWLY,,,,it aint a vented cap,,,,,

,,,,,The Autozone website shows that they don't carry a gas cap for our vans,,,,,

I don't get that much pressure and I have dual exhaust passing on each side of the tank, about 2" away on each side, but I thought, and could be wrong, is that vented also means it lets make up air into the tank otherwise a vacuum would form and starve the engine for fuel?? or are all caps this "one-way" flow design?  Modern fuel injected systems have a vent on the tank and a return line with pollution vapor from a canister instead of the cap for this purpose, its the old gravity fed systems that need a vented cap.  Makes sense that its to let pressure out...

,,,,,venting gas fumes into the air is now against the law,,,,,all newer caps are sealed going out,,,,,but they let air in,,,,the fumes on newer cars like from 1977 are vented to the charcoal cannister which are sucked into the intake manifold and burned,,,so that is a good piece of emission equipment to have,,,,, and our carbs vent into the air,,,,,,we have bought alleged vented caps,,,,they were not,,,,,so you modify one,,,,I would say if you get pressure out the filler neck when you remove your cap,,,that pressure is also working its way to the carb and making it run rich and add add fumes to the doghouse,,,,,,here is how they vent the gas tank on 1970 California Chevy vans,,,,,this cant be good,,,,,,or safe,,,,,,,

Hmmmm,

Drill press
+
gas cap
+
1 small drill
=
Problem solved

,,,,,,,,,,we gots to do what we gots to do,,,,,,

donivan65 wrote:,,,,,venting gas fumes into the air is now against the law,,,,,all newer caps are sealed going out,,,,,but they let air in,,,,the fumes on newer cars like from 1977 are vented to the charcoal cannister which are sucked into the intake manifold and burned,,,so that is a good piece of emission equipment to have,,,,, and our carbs vent into the air,,,,,,we have bought alleged vented caps,,,,they were not,,,,,so you modify one,,,,I would say if you get pressure out the filler neck when you remove your cap,,,that pressure is also working its way to the carb and making it run rich and add add fumes to  the doghouse,,,,,,here is how they vent the gas tank on 1970 California Chevy vans,,,,,this cant be good,,,,,,or safe,,,,,,,

Makes sense, that's why get a gas smell after its turn off. To test that, I'm gonna release the pressure as soon as I return to the garage next time out and see if that stops the aftersmell?

If you got pressure coming out of the gas tank that surely aint helping the smell,,,,,however,,,,,when you got a carburetor sitting on over a 300 degree manifold,,,that gas is going to boil,,,,,the only cure is a hamburger fan,,,,,with a twist timer to cool things off.....

I haven't driven mine enough in hot weather...but have yet to experience "boiling". Would be interested to hear from other members about their dog house heat issues. Here in the North of Vermont we don't get many super hot days. When I was a mere lad of 16 and had my first van (a'66 GMC) I never had the issue...but from what I understand todays gas has a lower boiling point?

I just bought me one of them there vented locking gas caps.....it has a spring loaded center diaphragm that lifts to release any pressure,,,,

Are the chevy tanks not vented through a small hose from the top like the ford ones. Just asking cus I don't no...but thinking they are. Which leads me to ask...Why put on a vented cap. Or does it have a vent tube that comes up the filler neck and vents under the cap. Again just asking cus I don't no.

The Chevy tanks have no vent,,,just a  line coming out for the fuel pump,,,,,,the filler neck has a bypass line to let the air come back out into the filler neck as the tank fills,,,,,,so you need a vented cap  because the pressure really builds up  in the tank from the exhaust flowing back.....but aint no law against asking questions,,,,,thats why we come here......