ok ...so my fuel system has 2 lines going to the pump of which i have been told one is a returen to prevent vapor lock..
i dont have a canister...

so it appears that i have a closed system..ie i cant see any sign of a vent..
how is pressure supposed to be vented if the system is closed or does it not matter if pressure is built up??

does a vented cap mean it lets pressure OUT ..or air IN as fuel level drops...or both..

i have noticed fuel leaking past my current cap on acceleration but as i said before when o release the fuel cap there is always a considerable pressure released

as memory serves, my dad had a 69 impala that had an aluminum appearing line which entered the truck sapce then possilby back down ( and irregularly shaped loop just in front of the trunk torsion bars...was that a vent pipe and should i have somethin similar??

what would hav been on the end of that loop or did it just end open beneath the trunk??

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Rich
Cocoa Beach, Fla
Team Chevelle #380
wa3men@aol.com
www.chevelles.com/showroom/70_SS_454.jpg (http://www.chevelles.com/showroom/70_SS_454.jpg)

[This message has been edited by 70 SS 454 (edited 01-08-2000).]

Vented caps draw in air as you lose fuel, and blow out vapor as pressure builds up.

Pressure does matter, as it causes vapor lock. If your system builds up pressure it will cause problems.

On the front passenger side of your tank there should be two outlets that are the air intake, and the two small hoses should be connected to a small canister.

You should also have a sealed gas cap, not a vented one. You'll loose lots of gas when you accelerate with a vented cap.

Before switching to a seal cap, I would first make sure your tank is vented from the holes in the front of it. If you tank is original, it will. You may want to remove it and check it to make sure it flows proper, and it isn't alowing fuel to leak out.

Hope this helps you somewhat.

Wes B.

the tank was replaced with new but dont know if it is nos or aftermarket i only see 2 lines coming out and running to the pump what cannister are you speaking of and exactly where will i find it??
under the trunk from beneath the car??

it does build pressure and as i stated since i only see 2 lines i presume it is a closed non vented system

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Rich
Cocoa Beach, Fla
Team Chevelle #380
wa3men@aol.com
www.chevelles.com/showroom/70_SS_454.jpg (http://www.chevelles.com/showroom/70_SS_454.jpg)

i just bought a new cap because mine said "vented" but i had a lot of pressure build up, the new cap holds SOOOOO much pressure that the tank actually expands,causing the filler neck to move upward and when i take the cap off (slowly) and release the pressure, it moves back down to it's original position. this sounds dangerous don't yall think could there be a blocked vent on top???

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members.aol.com/jnkb2cool/chevelle9.jpg (http://members.aol.com/jnkb2cool/chevelle9.jpg) members.aol.com/jnkb2cool/350.jpg (http://members.aol.com/jnkb2cool/350.jpg)
John Krenn
1970 malibu SS look-a-like well kinda
307/200-4r (soon to be 355)

Ft.Worth, Tx

Wes is right, all '70 Chevelle coupes had a vented tanks. California cars used a cannister, others did not. Wes has described the non-Calif. setup which is the most common. If your '70 came with NA9 option, it has the Calif. system and has(had) a carbon cannister located at the drivers side front of the engine compartment and a vent line from the rear of the car to the cannister. This system was on all Chevelles beginning in '71.

Fuel Cap Vent
Cars with evaporative emission controls used a vented cap with a pressure/vacuum valve. This was a safety vent in case the evaporative emission system got plugged(as in remove the canister and plug the hose). Even though they are vented, the fuel tank will build pressure or vacuum. As I recall, the pressure/vacuum cap has a tall skirt (1 1/4" or so) and the cam fingers extent down into the filler neck. If you don't mind a little gasoline taste, you can check these caps by blowing through them or make a cap checker from an old filler neck and a hand pressure/vacuum pump. Spec. is about 2 psi pressure, 3/4 psi vacuum.

Fuel Tank Venting
The fuel tank must be vented! It can build a lot of pressure especially if you're using very short or no tailpipes. It can also build vacuum to the point that the engine will starve for fuel.

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Fred Aldrich
Web Site: www.GeoCities.com/~69_chevelle (http://www.GeoCities.com/~69_chevelle)


[This message has been edited by Fred Aldrich (edited 01-09-2000).]