There should be two hoses from the filler cap assembly. One is for the tank vent and goes to the charcoal filter, the other is the rain drain for the cap. You need to vent the tank. Some remove the filter and just run the the hose down past the filter. If the filter is removed the hose to the intakes must be plugged to prevent a vacuum leak. You cannot plug the vent at the cap assembly or the tank will not vent. Looks like the nipple on the cap assembly is broken off?