How to Change the Heater Core in a Chevrolet S10by Eli Laurens; Updated November 07, 2017
Items you will need
Pliers or vice grips
A Chevrolet S10 truck uses an embedded heater core to circulate coolant through the radiator and heat the interior of the truck. This core can become clogged or leak, requiring replacement. The average backyard mechanic can replace a heater core in about two hours.
Remove the dashboard. The heater core in S10s is behind the main dashboard, and it must be removed to access the core. The front dash panel can be removed by unbolting several screws at the bottom of the panel, then pulling the panel away from the dash. Once this front panel is off, the primary dash can be unbolted from the firewall with four large bolts below the windshield. Several accessories may need to be taken out to unbolt the dash from the firewall, such as the radio or 4WD switches.
Drain the coolant from the core. There is a drain plug at the bottom right of the primary radiator that will empty the coolant system when unscrewed. Some residual coolant might remain in the core, but it will come out when the core is taken out later.
Uncouple the core from the coolant lines. These hoses are directly behind the oil filler tube and will come loose when squeezed with pliers. They are of different sizes, and it won't be necessary to mark them for later re-attachment.
Unbolt the core from its mounts behind the removed dash panel. Several bolts might be difficult to reach, but once free from the mounts, the core assembly slides up and out toward the interior of the truck. Lay a towel down when removing the core, as come coolant might splash out.
Insert the new heater core and bolt it into the mounts. The openings will fit into the holes provided in the firewall.
Reconnect the heater core hoses, and re-attach the dashboard and front panel.
Fill the coolant system with a 50/50 mix of antifreeze and water. Tighten the drain plug on the radiator, then fill it. Crank the truck up with the heater system on its maximum setting and continue to pour coolant into the radiator until it will not take in any more.
Use distilled water instead of tap water for added coolant stability.
Do not allow coolant/antifreeze to drip openly, as animals may drink it.