Where is the other end of the steam vent hose connected?
Pretty sure you have air trapped in your system. Have you removed the cap and burped the system by squeezing the hoses etc while the car is running and hot? Other trick is to raise the front of the car while it is hot and running.
Get one of those low cost IR temperature meters to measure the temperature of your coolant system. One like this:
http://www.ebay.com/itm/Non-Contact-...QAAOSw241Ya7C0
Don