The fuel tank filler cap (9030)
is sealed cap with a built-in pressure-vacuum relief
valve and:
Fuel system vacuum relief is provided after negative 1.7 kPa (0.25 psi)
Pressure relief above 14 kPa (2 psi)
Under normal operating conditions, the fuel tank filler cap
operates as a check valve, allowing air to enter the fuel tank (9002)
as fuel is used, while preventing fuel vapors
from escaping the fuel tank
through the fuel tank filler cap
.