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)