More than likely it is your EVAP valve malfunctioning. Locate the valve on the radiator support sort of underneath the cold air intake. I should have two hoses and a Bosch type electrical connector attached. Trace the hose to the intake manifold and make sure it is not cracked, dry rotted, leaking etc. then remove the valve and using a vacuum pump, check each side to see if it will hold a vacuum. If not, replace. You can also apply 12volts to the spade terminals and listen for a click of the solenoid.