Fuel Filler Caps

[David Doddek]

>Every fuel filler cap I have ever seen on an American car was designed
>to let air into the tank but not back out. Therefore you could have
>pressure in the tank (split muffler next to the tank produces
>interesting results like gas two feet in the air out of the carburetor
>bowl vents and an airborn filler cap when removed) but never a vacuum.
>
>
Although the gas cap is not vented, the tank should be.  On older cars, some
had a vented cap and others vented through a tube to the outside of the car.
My 96 Fairlane has such a tube.  On newer cars, it is vented through the
charcol canister.  The charcol canister may have some back pressure, but not
enough to blow fuel out of the carb.  Although the heat of direct exhaust
will generate a rapid buildup of fumes.  Normally if you have too much
pressure, or vacuume, in a tank, the part of the venting system is plugged.
It is possible that some cars were made with no vent to let fuel vapor
escape.  But I would prefer my car not be that way.

David Doddek        pantera at pobox.com        217-422-3722
69 EFI Fairlane, 89 T-bird SC, 74 Twin turbo NOS EFI Pantera (I like to go fast)