Replacing the heater core can be a complicated and time-consuming repair because of its hidden location and tight quarters. The heater core in your 1978 Ford F-150 is radiator-like unit used to heat the interior of your truck. It's mounted behind the passenger's side dashboard. Heated engine coolant flows from the radiator to and through the heater core, causing the unit to heat up. A blower fan forces air through the core and through the cabin air vents when the heater is turned on. A malfunctioning or clogged heater core will leak anti-freeze onto the inside floorboard on the front passenger's side.
Tools Used: Safety glasses, Rubber surgical gloves, Crescent wrench, Recycling container, Phillips screwdriver, Socket wrench set, Channel-lock pliers, 1 gallon antifreeze
Replace Heater Core
Removing the Heater Core
Open the hood and disconnect the negative battery cable using a crescent wrench. Place the recycling container underneath the radiator drain plug to capture the drained coolant.
Put on your safety glasses and rubber surgical gloves. Remove the radiator fill cap. Loosen the radiator drain plug using a socket wrench to drain the radiator coolant. Once drained, replace the drain plug and tighten. Leave the recycling container in place to catch the excess coolant sitting in the lower radiator hose.
Disconnect the lower radiator hose where it attaches to the radiator by loosening the squeeze-type hose clamp. Allow the excess fluid in the hose to drain into the recycling container.
Locate the heater core intake and return hose lines, which are protruding through the firewall on the passenger's side of the engine compartment. Remove the hoses from the dual core nipples by depressing the release clip with a flat-head screwdriver while turning the hose counterclockwise by hand.
Remove the screws located under the front dashboard that holds the dashboard soft cover in place, using a Phillips screwdriver. Locate and remove the mounting bolts attaching the dashboard frame to the interior firewall. This will allow the dashboard assembly to swing free, allowing you enough clearance to access the heater cover and core, down from the windshield.
Remove the outer mounting bolts holding the heater core cover in place to access the actual heater core. Disconnect the heater core ground cable strap that attaches to the interior firewall with a single bolt. Detach the interior heater core intake and return hoses from the core nipples by releasing the squeeze clamps using channel-lock pliers.
Take out the mounting bolts holding the heater core in place and maneuver the unit out and up toward the windshield. Be careful not to spill the coolant in the heater core.
Installing the Heater Core
Install the replacement heater core in reverse, down from the windshield and behind the dashboard. Mount and bolt down the new heater core. Reconnect the cable ground strap and coolant hoses. The hose nipples have different diameters so that the coolant hoses can't be crossed incorrectly.
Reconnect the dashboard frame and cover in reverse of the removal, and tighten all of the mounting bolts. Reconnect the coolant hoses located in the engine compartment at the firewall core nipple. Connect the lower radiator hose to the radiator and secure it in place using its squeeze-type hose clamp.
Refill the radiator with a 50/50 mixture of new anti-freeze and water. Reconnect the battery and start the engine. Run the engine for several minutes to flush out any trapped air within the cooling system while checking the connections for leaks.
Test the replacement heater core by turning the heater on and then off continually, observing changes in temperature.
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