1. too loose (do you have a spring loaded tensioner or is it manual?)
2. vapor locked?  probably not.  Did you let the engine warm up?  you may have a blockage or bad heater control valve.  Try flushing your system (get the prestone flush kit, it splices into your heater line and you hook a garden hose to it and force flush the system.  you can also get some stuff to add that will break appart the corrosion prior to flushing)