changed out the heater core on the 98 tahoe today, and noticed what i think is just a vent line, but not sure. it's in line with the two water lines going into the heater core, towards the passenger side. on the firewall, it's bout 1/2 inch diameter with a 90 degree rubber elbow attached. i see no where on the motor it could go, so i assume it's just a vent line, but would like to be sure. everything works great, motor runs great, so i don't believe it's anything to worry about. just want to be sure.

 

A picture may help.