How to Replace a Heater Core in a 1998 S10 Blazer

The heater core in your 1998 Chevy S10 Blazer is radiator-style unit, used to heat the interior cabin of your truck. Engine coolant flows, continuously, from the lower radiator hose to, and through, the heater core, causing the unit to heat. An attached fan forces heated air through the heater core fins and into the cabin when the heater is on. A faulty heater core leaks coolant onto the inside floorboard on the front passenger’s side, signaling that a replacement is needed. Replacing the heater core can be a complicated, time-consuming repair, as it is hidden behind the passenger’s side dashboard.

Heater Core Removal

Put on your safety glasses and rubber gloves. Raise the hood, and disconnect the battery cables, using the socket wrench. Place the recycling container underneath the radiator drain plug to capture the drained coolant.

Take off the radiator fill cap to relieve the pressure in the cooling system. Loosen the radiator drain plug, using a socket wrench, to drain out the radiator fluid, and re-tighten it. Leave the recycling container in place to capture the excess coolant sitting in the lower radiator hose.

Disconnect the lower radiator hose, where it attaches to the radiator. Drain the excess fluid into the recycling container.

Locate the heater core intake and return hose lines that protrude through the passenger’s side firewall of the engine compartment. Detach the hoses from the core nipple by depressing the release clip with a flat-head screwdriver, while turning the hose, counterclockwise, and pulling it off, by hand.

Take out the Torx screws, located under the front dashboard lip, which hold the dashboard soft cover in place. Remove the underneath mounting bolts, which attach the dashboard frame to the interior firewall. This allows the entire dashboard assembly to swing free, providing access to the heater cover and core assembly downward from the windshield.

Remove the heater core intake and return hoses from the core nipple by releasing the squeeze clamps, using channel-lock pliers. Squeeze the clamp, and slide and twist the hose, as you pull them off.

Remove mounting bolts holding the heater core cover in place, using swivel sockets and extensions. Take out the mounting bolts attaching the heater core to the interior firewall. Disconnect the electrical wiring harness. Maneuver the old unit upward and outward, toward the windshield.

Heater Core Installation

Install the replacement heater core downward from the windshield. Mount the unit, using its bolts.

Reattach the return and intake hoses. The hoses are different sizes, so you can not incorrectly cross them. Reattach the heater core cover and bolt in place. Reconnect the electrical wiring harness. Reattach, mount and bolt in place the dashboard frame and soft cover.

Reconnect the intake and return hoses, located in the engine compartment, at the firewall. Snap the hoses into the core nipple. Connect the lower radiator hose to the radiator, and secure it, in place, using the squeeze-type hose clamp.

Fill the radiator with a 55/45 mixture of new antifreeze and distilled water. Reconnect the battery, and start the engine. Allow it to run for ten minutes to flush out trapped air within the cooling system.

Test the replacement heater core by turning the heater on and off, continually, while observing the change in temperature. Check for leaks at all hose connections and the passenger’s side floorboard.