flushing a heater core

red67gts said

why not just swap hoses going into firewall and reverse flow through heater core and maybe dislodge any obstruction

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And plug up the coolant passages in the engine.

flush the core by putting water into that hose that stays cold. That will be the direction opposite of the way it flows through the system.

EDIT: I should have read the whole thread before posting that.

HEMIPAR: technically it does not matter which way the coolant flows through the heater core. You can switch the hoses around and it will work the same. It sounds like you have a bad thermostat.