Heater on my ZT-T V6 was blowing cold.
Found a hill and managed to find a bleed valve on the tubing and left it running with the coolant top off.
Lots of popping and hissing later it appeared sorted.
However - having traced the car since, it was one of the very very few V6s to suffer HGF shortly after, and I think my issues with the heater/coolant system and tubing were linked to that.
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