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With or without A/C, in either case the upper hose is your return (waterpump fitting)
A quick test is to bring the water temp up to normal. Check the inlet hose (from intake) is it hot? should be... Next check the outlet hose (to waterpump) is it warm? Is it Hot? or is it cold.
If its hot, your not drawing air across the heater core (check blend door). If its cold you have a restriction in the water flow (heater core/waterpump/intake)
Check for crimped heater core pipes, check for good flow coming from the 5/8's hose (from the intake).
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