How to replace heater core
Disconnect the negative battery cable. Remove the water hoses attached to the heater core at the firewall. Remove the bolts that attach the air plenum chamber to the firewall, located just below the windshield wiper motor. Remove the nut that holds the top left-hand corner of the air conditioning evaporator core. Remove the screws securing both of the instrument panel insulators to the firewall and carefully remove the insulators. Unhook the vacuum hoses and feed them through the firewall and into the passenger compartment. Go inside the Grand Marquis and remove all of the retaining screws from the dashboard. Remove the screws from the passenger-side door. Then, remove the trim panel with the door-panel removal tool. Disconnect the power window and lock wiring harness and feed the loose end through the access port. Pull the dashboard back as far as possible (without having to disconnect the instrument panel wiring harnesses). Unhook the vacuum hose harness, located at the heater duct under the passenger's side of the dash. Disconnect both inlet (white) hoses from the heater motor. Unscrew the guard panels and disconnect the plastic retainers with a screwdriver. Remove the two screws at the back of the air plenum chamber and the nuts located at the sides of the bottom flange. Remove the air plenum chamber by sliding it slightly backwards, rotating it slightly to the right and then down while pulling gently on the dashboard panel. Loosen the four screws that attach the heater core to the top of the air plenum chamber and remove. Replace in opposite manner.
Of the heater cores I did it help to have a good variety of 1/4" sockets,swivels, extensions, flex shafted nut driver, etc. Without them don't start the project.
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