Chances are it is a vacuum hose. If it dealt with the heater, it would most likely be leaking coolant. If you can upload a pic, it would help.
December, 31, 2012 AT 12:02 PM
Describe heater not working? Vacuum operated controls get vac feed from small hose with plastic line 1/8 inch down behind intake at firewall. That hose you describe sounds like pcv hose. Is it left or right of oil cap?