Scott,
If the radiator cap has lost spring tension that would cause system pressure to escape
Into the overflow or steam out through the cap. If your coolant level remains full then it is
Likely good. The system needs to be sealed tight to work effectively.
Buy a HAYNES repair manual at the parts store for around $ 16 it will guide you with
Info & pictures on almost everything for your van.
Also check the temperature of the hoses going in and out of the heater core that they
Are hot ensuring good flow through the core.
This site might help you a bit too:
http://www.misterfixit.com/