Change heater core water pump one pipe get hot

Try some comma's?
my wild guess, is you are trying to fix a dead cab heater, right?
and if so, can be caused by 5 things wrong. total.
1 engine coolant not hot (180F or more)
2; fan dead.(cab heater fan)
3:or core , coolant path blocked. (seems you)
4; core exterior core blocked with leaves or pin needles. (sucked in)
5: optional core valve , stuck closed. (cable bad, etc)\



one pipe hot , means ,? core pipe 1 is hot, and other is dead cold?
this flow blockage , bingo !

if pipe 1 is 180F and other is 120f this is normal the heater is heating air in the cab and is dropping due to this heat exchange. (normal)
so why not do the 2 year full flush service now. (skipped i bet)
core hoses:
remove the 2 hoses on engine side, never core side (easy damage)
then back flush both hoses. with a garden hose.
if there is a valve there, test it and make sure it's opening... and free flowing.
and that both engine water ports are open too, from heater hose
to engine. (yes, they pack up too)