yes the cap should be loose to allow it to automatically vent any air, it valve is most likely clogged with sluge or scaled causing it to either stick or not seat correctly
close that cap and allow the boiler to run, it should be ok, but if you start getting unexplanined boiler lockouts all the time it may need to be replaced (then again, shouldnt be big pockets of air in your system if its correct)
T
May 2008