[sandorjoben]

The new blower motor may move more CFMs of air than the one that died. If the aforementioned hose test passes(it'll tell you if the core is plugged) and the flap that directs airflow between "HEAT" and "VENT" positions still functions properly(AND seals properly), it may very well be that the new motor is moving more air and some of it is just passing around the the core.

If you think this may be the case then you will need to make sure that any space immediately surrounding the heater core is properly sealed so that the air MUST pass through the fins of the core. Adhesive backed foam weatherstripping will do the job of filling any gaps.

If the heater core is broken, then I agree, find a copper replacement if you can over an aluminum one, the aluminum ones are touchy to install because they're so soft.