How do I change the heater core on my 1990 GMC Suburban?

Answer 1

Heater Core
REMOVAL & INSTALLATION
CAUTION
Never open, service or drain the radiator or cooling system when hot; serious burns can occur from the steam and hot coolant. Also, when draining engine coolant, keep in mind that cats and dogs are attracted to ethylene glycol antifreeze and could drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantities. Always drain coolant into a sealable container. Coolant should be reused unless it is contaminated or is several years old.


Front
1988–91 R/V SERIES WITHOUT AIR CONDITIONING
Disconnect the negative battery cable.
Disconnect the heater hoses at the core tubes and drain the engine coolant. Plug the core tubes to prevent spillage.
Remove the nuts from the distributor air ducts in the engine compartment.
Remove the glove compartment and door.
Disconnect the air-defrost and temperature door cables.
Remove the floor outlet and unfasten the defroster duct-to-heater distributor screw.
Remove the heater distributor-to-instrument panel screws. Pull the assembly rearward to gain access to the wiring harness and disconnect the wires attached to the unit.
Remove the heater distributor from the truck.
Remove the heater core retaining straps and remove the core from the truck.
To install:

Install the heater core. Be sure that the case-to-core and case-to-dash panel sealer is intact.
Position the heater distributor. Connect the wires attached to it and then install the heater distributor-to-instrument panel screws.
Install the floor outlet and tighten the defroster duct-to-heater distributor screw.
Connect the air-defrost and temperature door cables.
Install the glove compartment and door.
Install the nuts into the distributor air ducts (in the engine compartment).
Connect the heater hoses at the core tubes.
Connect the negative battery cable. Fill the cooling system and check for leaks.
1988–91 R/V SERIES WITH AIR CONDITIONING
Disconnect the negative battery cable.
Disconnect the heater hoses at the core tubes and drain the engine coolant. Plug the core tubes to prevent spillage.
Remove the glove compartment and door.
Disconnect the center duct from the defroster outlet duct.
Disconnect the center, lower air distributor and the center air outlet ducts.
Disconnect the temperature door cable.
Remove the nuts from the 3 selector duct studs that project into the engine compartment.
Remove the outlet duct-to-instrument panel screws. Pull the assembly rearward to gain access to the wiring harness and disconnect the wires and vacuum tubes attached to the unit.
Remove the heater distributor from the truck.
Remove the heater core retaining straps and remove the core from the case.
To install:

Install the heater core. Be sure that the core-to-core and case-to-dash panel sealer is intact.
Position the heater distributor. Connect the wires attached to it and then install the heater distributor-to-instrument panel screws.
Install the floor outlet and tighten the defroster duct-to-heater distributor screw.
Connect the temperature door cable.
Connect the center and lower air distributors and the center air outlet ducts.
Connect the center duct to the defroster outlet duct.
Install the glove compartment and door.
Connect the heater hoses at the core tubes.
Connect the negative battery cable. Fill the cooling system and check for leaks.
1988–90 C/K SERIES
Disconnect the negative battery cable.
Remove the coolant overflow bottle.
Drain the cooling system.
Disconnect the heater hoses at the core tubes.
In the engine compartment, remove the heater case-to-firewall screws.
Disconnect the antenna cable at the mast.
Remove the glove box.
Disconnect the wiring harness at the engine's Electronic Control Module (ECM).
Remove the ECM and bracket.
Remove the right side kick panel.
Remove the right side lower dash panel bolt and nut.
Remove the heater case mounting bolts.
While lifting the instrument panel slightly, remove the case assembly.
Lift the core from the case.
To install:

Lower the core into the case.
Position the case assembly against the firewall.
Install the heater case mounting bolts.
Install the right side lower dash panel bolt and nut.
Install the right side kick panel.
Install the ECM and bracket.
Connect the ECM wiring harness.
Install the glove box.
Connect the antenna cable at the mast.
Install the heater case-to-firewall screws.
Connect the heater hoses at the core tubes.
Fill the cooling system.
Install the coolant overflow bottle.
Connect the battery cable.
1991–92 R/V AND 1991–98 C/K SERIES
Disconnect the negative battery cable.
Drain the cooling system.
Remove the instrument panel glove compartment.
Tag and unplug all electrical leads as necessary.
Remove the center air distribution duct from the floor.
If necessary, remove the ECM and mounting tray.
Remove the hinge pillar trim panels.
Remove the blower motor cover and then remove the motor.
Remove the steering wheel and tilt back the instrument panel slightly.
Remove the coolant overflow tank. Disengage the heater hoses at the core connections.
Unfasten the attaching screws and nut and then remove the heater case.
Remove the seven screws and lift off the heater case cover.
Remove the fasteners and brackets that hold the heater core to the case and lift the core from the case.
To install:

Install the heater core in the case.
Install the heater case bottom plate. Make sure there is a good seal.
Install the heater case into the truck. Tighten the 4 lower screws to 17 inch lbs. (2 Nm); the upper screw to 97 inch lbs. (11 Nm); and the nuts to 25 inch lbs. (2.8 Nm).
Connect the heater hoses at the core tubes.
Install the coolant overflow tank.
Install the instrument panel and the steering wheel.
Install the blower motor and cover.
Install the trim panels and if removed, the ECM. Install the air distribution duct and reconnect all electrical leads.
Install the glove compartment. Fill the cooling system and connect the battery cable.
Rear
1988–91 MODELS
Disconnect the negative battery cable.
Drain the cooling system.
Disconnect the heater hoses at the core tubes.
Disconnect the blower motor wiring harness.
Remove the blower motor clamp.
Remove the motor attaching screws and lift out the motor.
Unfasten the upper-to-lower case half screws and remove the upper case half.
Remove the core seal.
Lift out the core.
To install:

Install the heater core.
Install the core seal.
Install the upper case half and then tighten the upper-to-lower case half screws.
Install the motor and then tighten the attaching screws.
Install the blower motor clamp.
Connect the blower motor wiring harness.
Connect the heater hoses at the core tubes.
Fill the cooling system.
Connect the negative battery cable.
1992–98 MODELS
Disconnect the negative battery cable and drain the engine coolant.
Remove the right rear quarter panel trim and then remove the panel itself.
Remove the right rear wheelhousing liner.
Disconnect the heater hoses from the core.
Unplug the electrical lead. Remove the drain valve.
Remove the heater module.
If necessary, remove the blower motor.
Remove the heater case cover.
Remove the heater case cover and then remove the heater core.
To install:

Install the heater core and then install the heater case cover.
If removed, install the blower motor.
Remove the heater module.
Attach the electrical lead. Install the drain valve.
Connect the heater hoses to the core.
Install the right rear wheelhousing liner.
Install the right rear quarter panel and then install the trim.
Connect the negative battery cable and fill the engine with coolant.

Answer 2

man this is a job ..take the dash out then the heater box ,,dont forget to drain the radiator if you are not mechanicly incline let a shop do it for you