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IT could be one of several things: possible the blower fan motor is bad; or the valve that lets the hot water in from the engine to the heater core is not opening; or the door that lets the air flow past the heater core and onward to your vents is not opening. All these are controlled by your heater switch, via relays and vacuum lines. If the blower motor works well when you put power to it directly, but not when it is hooked to the car, a connection in the switch or a relay may be bad. if you get little or no effort from the blower motor when you hook it directly to a good battery, it's the blower motor itself gone bad. If you get airflow but not heat, it's the device which diverts the hot water into the heater core. and if its the door to let air flow over the heater core, it could be a relay, a vacuum hose or the switch.
Couple of possibilities here. One is the heater core bypass valve that allows the coolant to flow to the heater core. Make sure that the vacuum tube has not come off of it. Should be under the hood just on top of the blower motor assembly. Let the engine get hot and check to see if the hoses going to the heater core are getting hot coolant flowing through them.
Does the air flow through the defroster, front vents, and floor vents when they are selected? If not, this can indicate a vacuum leak on the HVAC control panel or a failure of the switch.
Could be faulting acutar motor for the hot air door.
Heater Core Cover
A/C Defroster Seal
Upper HVAC Module Case
A/C Valve Lever
Mode Valve Actuator
Heater/Defroster Valve Lever
Mode Valve Cam
Lower HVAC Module Case
Rear Heater Duct Seal
Temperature Valve Actuator Screw
Temperature Valve Actuator
HVAC Module Wiring Harness
Blower Motor Resistor/Relay Assembly
Blower Motor Acoustic Insulator
Blower Motor Fan
Passenger Compartment Air Filter Access Door
Evaporator Block Seal
Passenger Compartment Air Filters
Air Inlet Valve
Air Inlet Housing
Air Inlet Housing Seal
Air Inlet Actuator
Heater Core Pipe Seal
Or the blower motor works but there is not very good air flow out the vents? If so it was pretty common on the older GM's for the evap core to leak and cause a bunch of debris/dust/dirt to stick to it causing airflow to slowly be limited as it became covered in dirt. Sometimes you can remove the blower motor and clean the evap core to regain some air flow.
Does the heater blow cold air from the vents or do you mean the there is no airflow from the vents? cold air from the dash vents when the heater control valve is set fully hot could be an air pocket in the cooling system. The radiator sits slightly lower than the engine causing air pockets to get trapped in the engine when you replace the coolant. you'll have to purge the cooling system to restore flow to the heater core. No airflow (blower motor inop) this could be a bad blower motor transistor. pin test the blower motor circuit.
I believe you have one of two problems either your heater core is plugged up and not allowing enough flow or the temperature blend door located under the dash is not moving.The heater core is easy to check,A quick check of the heater core is with the engine running and with the blower motor off or on low carefully chck to see if both heater hoses are equally hot if not remove them from the heater core and use a garden hose to backflush it the blend door can be more of a hassle as it can be operated by a motor or vacuum servo and even a simple cable these also have been known to break but with the blower on high change the temp control from cold to hot and see if you can hear a change in the sound of the blower and sometimes you can hear the door shut it more than likely the door is ok but you will need to check it and see.