The Chevrolet Blazer uses a coolant-heated radiator core mounted in the dashboard to provide heated air into the passenger compartment. This heater core can become clogged and leak, as it is usually the lowest point in the coolant system and debris settles into it. Replacement of the heater core will take the average backyard mechanic about two hours to complete.
Remove the front dash panel by turning the lower screws in a counter-clockwise direction, then unscrewing the single bolt on the lip of the instrument cluster and pulling the panel out. Remove the rear wiper switch and the 4x4 switches, if applicable. Lowering the tilt wheel will allow the panel to be totally removed.
Remove the radio, headlight switch module and glove box by turning their screws in a counter-clockwise direction.
Remove the main dashboard by unbolting it at the front, between the windshield and firewall. There are six to eight bolts holding it in, and, once they are removed, the dash can be pulled away from the firewall. Be careful not to pull too much, as the wiring harness could become damaged.
Drain the coolant system by turning the drain plug on the lower left corner of the radiator and allowing the fluid to fall into the drain pan. Uncapping the radiator can reduce the internal pressure.
Remove the heater core hoses, on the upper left of the engine bay, by pinching their hose clamps with a pair of pliers and pulling them off. Some models will have screw-type hose clamps; turning them counter-clockwise will loosen them. Age and heat may have stuck the hoses to the core nipples, and the hoses can be cut right at these nipples if necessary. Leave enough hose to reconnect the new core.
Unbolt the heater core's mounts with a socket wrench in a counter-clockwise direction. Once the bolts are free, the heater core can be manoeuvred toward the windshield and slid out. Some coolant may splash out, so have a towel handy.
Replace the heater core with a new one by sliding it into place and securing the mounting bolts.
Reconnect the heater core's hoses, turn the radiator's drain plug clockwise to close it, then refill the coolant system with a 50/50 per cent mix of coolant and water.
Reconnect the dashboard to the firewall, then replace the front dash panel and fasten its instrument cluster screw and bottom bolts.
Crank up the truck and add coolant until the heater core is filled and blowing hot air.
Replace the radiator cap.
Using distilled water raises the cooling ability of the system and prevents corrosion.
Do not allow animals or children to come into contact with the drained coolant.