Which direction does my vented gas cap vent
Thread Starter
Safety Car
Joined: Jun 2004
Posts: 4,023
Likes: 1,181
From: La Mesa CA
Which direction does my vented gas cap vent
I have a 73 with a gas cap that says vented
Does it vent in both directions
or just let outside air in?
It seems to hold pressure that builds up
inside the tank. Is that normal
or is it plugged in some way?
Does it vent in both directions
or just let outside air in?
It seems to hold pressure that builds up
inside the tank. Is that normal
or is it plugged in some way?
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
A C3 vented cap is just cap w/o the gasket. And you will smell fumes in the garage.
Now a non-vented cap will hold pressure but has a pressure relief valve built in and you can actually hear it fart on hot days when pressure builds to high. But these were used on later models like my '74 that have the charcoal canister installed on the tank vent. The charcoal vapor canister also provides vacuum relief. So if you have a non-vented cap on a non-vented tank you may have a over-pressure relief but no vacuum relief to prevent the tank from collapsing as the fuel is sucked down.
Hope that helps more than it confuses you.
Now a non-vented cap will hold pressure but has a pressure relief valve built in and you can actually hear it fart on hot days when pressure builds to high. But these were used on later models like my '74 that have the charcoal canister installed on the tank vent. The charcoal vapor canister also provides vacuum relief. So if you have a non-vented cap on a non-vented tank you may have a over-pressure relief but no vacuum relief to prevent the tank from collapsing as the fuel is sucked down.
Hope that helps more than it confuses you.
Team Owner
Joined: Sep 2006
Posts: 31,279
Likes: 4,372
From: Westminster Maryland
Hi G,
A vented gas cap certainly should have a gasket. The gasket provides the seal between the filler neck rim and the cap.
The pressure vacuum release gas cap is closed during all normal operating modes. The cap is designed to relieve pressure or vacuum, (through an internal relief valve in the cap), in the tank to prevent failure due to malfunction of the button relief valve or the vapor separator system.
The proper cap has a 'second' set of 'ears' that provide a slow bleed down of tank pressure when the cap is turned to begin removal. Temperature and fuel volatility contribute to the frequent pressure build up in the tank.
With a properly operating tank, cap, and ECS vapor canister you should smell no fumes in the garage.
Regards,
Alan
A vented gas cap certainly should have a gasket. The gasket provides the seal between the filler neck rim and the cap.
The pressure vacuum release gas cap is closed during all normal operating modes. The cap is designed to relieve pressure or vacuum, (through an internal relief valve in the cap), in the tank to prevent failure due to malfunction of the button relief valve or the vapor separator system.
The proper cap has a 'second' set of 'ears' that provide a slow bleed down of tank pressure when the cap is turned to begin removal. Temperature and fuel volatility contribute to the frequent pressure build up in the tank.
With a properly operating tank, cap, and ECS vapor canister you should smell no fumes in the garage.
Regards,
Alan
Last edited by Alan 71; Jul 26, 2016 at 06:34 AM.
