2004 GMC Envoy Evap Emission Control Service Port

I have an OBD code P0442 on my Envoy. I replaced the gas cap and reset the code but the service engine soon light came back on after about 30 miles driven. I noticed that there is a what looks like a vacuum port that is located on top of the engine, just in front of the TBI. Is this port supposed to be plugged or capped off? Mine is open. I suspect that this open port is causing my problem. Please help. Thanks

Welcome to AF. It is a test port to introduce low pressure into the evap system for test purposes. There may be a cap on it like a tire valve cap, but that is not for sealing purposes. If you cannot see any leaks around various hoses in the system, a smoke machine may be required to track this down.