Ive got a question about a 70 nova, my friends gas tank has no vent in it, i would think that there needs to be one in a gas tank...Does it need a vented gas cap or what is the deal? thanks graemlins/beers.gif

This same topic drives me insane.

I have a 70 chevelle with a regular gas tank with a mech, pump and no vent. I have tried multiple vented gas gaps, drilling small holes (bad idea), and lots of other stuff.
never mind the fact that sometimes gas manages to get by the cap under hard acceleration and spill out, but the tank builds up pressure like its gonna blow. On hot days its worse, when there is less gas its worse, when cold gas from underground is pumped up I have to vent it by hand too, once I filled it, the gas expanded, and leaked out from the cap.
When I go to get gas I have to unscrew the cap very very slowly so I don't get a rush of gas in my face.
When the car starts to idle/normal driving bad on a hot day, I have to stop, pull over, open the cap, woosh, and idle problem is fixed.

matt

Ya well, with my friends nova, sometimes pressure will shoot out when you take the cap off, sometimes it wont...But i think at high speeds under high load it has negative pressure in the tank, and may be causing a shortage of fuel to the engine...so if anyone could tell us if a vent to the gas tank is neccesary, i would be very appreciative graemlins/beers.gif

p.s my chevelle has a vent, and all lawn mowers ive seen have em in the cap...so that would make me think you'd need one.

You definitely need a vent on the tank. Some stock setups have a small tube that vents the filler neck, like my 66 Nova. Some use a vented cap, some vent to a charcoal canister.

Yes the tank needs a vent. As said, there are several schemes. If the filler is above the tank, a vented cap will work. If the filler is low, the tank should have a vent somewhere on it and an unvented cap is used.

http://www.chevelles.com/forum/ultimatebb.php?ubb=get_topic;f=1;t=006869#000001

http://www.chevelles.com/forum/ultimatebb.php?ubb=get_topic;f=1;t=006378#000002


Gas cap CST 6807

Autozone $3.99

Remove valving mechanism by bending retaing arms and cut relief spring in half then reassemble

will it leak? could you just use a hose coming out of one of the lines cast inot the tank, and then put one of the manual tranny or rearend breathers in it?

I will somewhat disagree with everyone here... Your tank needs a certain amount of pressure - not a constant vent though. As the temp increase the vapor pressure of the fuel increases. You want the tank to be pressurized because you dont want to lose the volatile hydrocarbons in the fuel. If you vent them your fuel will begin to lose its "ummph" and go stale much quicker. The tank needs to be kept at a pressure thats above the vapor pressure of the fuel. This probably wouldnt be an issue in a daily driver car but if you have a weekend cruiser where the gas sits for weeks at a time, you run the risk of the fuel going bad. You also risk moisture getting into the tank over time.

My gas tank (not a chevelle) has a pressure valve built into it from the factory. When the gas tank pressure reaches a certain point it will purge the excess. Ever notice a lot of new cars will give you a check engine light if you don't tighten the gas cap all the way? Thats why (and because gasoline vapors escaping into the atmosphere is bad emissions-wise).

those vented gas caps do not work!!!! been there done that. My 1968 builds up serious presure out here in the desert where I live where it can reach 110 in the shade. Just the other day my tank had so much presure in it, it flooded my engine and couldn't get the engine to start. Gotta do something about this. Been a ongoing problem for years with my el camino. Can seem to solve it... :confused:

Originally posted by michaelem:
those vented gas caps do not work!!!! been there done that. My 1968 builds up serious presure out here in the desert where I live where it can reach 110 in the shade. Just the other day my tank had so much presure in it, it flooded my engine and couldn't get the engine to start. Gotta do something about this. Been a ongoing problem for years with my el camino. Can seem to solve it... :confused: I've never had a problem with my El Caminos both have vented caps. Granted it's not as hot here in Pomona but we see a lot of over 100º days. Neither my 66 or 67 have charcoal canisters. I don't know if a 68 does, but that might be something to look at.

Ive got a question about a 70 nova, my friends gas tank has no vent in it, i would think that there needs to be one in a gas tank...Does it need a vented gas cap or what is the deal? thanks graemlins/beers.gif


I have the same problem with my 1970 El Camino. I have about a dozen gas caps, both vented and non-vented, and NONE will reduce the pressure in the tank, by itself. I get the same whoosh as everyone else does, when I open the cap. I, even, tried NON-Ethanol gas @ $4.49/gal. and that doesn't seem to help, either.
Don Heflin, Naples, FL @ heflinhunter@yahoo.com

When I did my 68 Velle, 70 and 71 Monte, I used 71 EEC tanks and just triple looped the vent hose above the tail pipes and no more SWOOSH when I loosen the cap.

Fuel tanks prior 1971 were vented through the fuel cap on our old Chevys. There's a one-way check valve in the FC that allows air into the tank to replace spent fuel. Fuel or fumes would not be allowed to escape for obvious reasons.

Rattle your fuel cap. If it rattles similar to a PCV valve, and if the rubber gasket is properly sealing, your fuel tank is vented.

Yes, the tank can pressurize a bit from the heat from the exhaust, agitation from driving, and changes in atmospheric pressure. Perfectly normal, and why sometime you'll experience a "whoosh" when you remove your fuel cap.

After 1971, the EPA decided that this "whoosh" wasn't good for the environment and the closed evac system was employed which included a fuel separator at the tank, a charcoal can, a bit of vacuum from the induction system, and the plumbing.

FWIW

The 66 cars have an expansion line inside the trunk by the RH wheelhouse too

this has prob caused more problems than that frame mount/motor mount thing, lol-on some cars i have done everything possible to relive pressure-wierd, on some the fuel expands and blows out, some of the same model dont