not sure youre vehicle, but my buddy's tj had a air bubble in the heater core. we had to loosen the top heater hose (be careful of course its hot) to vent it.
on cj's the heat just sucks without a blower upgrade.
could have some crap in the heater core, might want to flush the whole system.
sometimes leaves and stuff can get in the plenum and block air.
i know you said its full, but the heat is the first thing to go as level drops.
Josh


80 CJ-7.
It runs