That is impossible without knowing exactly the vehicle you are working on. Basically you just have to remove enough of the dash in order to gain access to the heater core. This is why replacing a heater core is a very challenging repair. It is so very labor intensive and very expensive if done by a dealer. I know of no site that has detailed instructions. Go slow, take your time, and you can do this.
Drain the radiator. disconnect the heater hoses as they go through the firewall. From inside of the cab, remove the heater core housing, then the core. Reverse the procedure to finish the installation.
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Bolt the heater core into its proper position. There are four retaining bolts. Connect the water lines to the heater core. Fill the cooling system with coolant.
The heater core itself, should be under $50. Installation aprox $300 to $400.
When removing and replacing a heater core, it is important to have a diagram. This diagram for the given can be found in its maintenance manual.
Get a manual on your car from DISCOUNTAUTOREPAIRMANUALS.COM
Drain the radiator. Disconnect the heater hose connections at the fire wall. loosen the dash screws and pull it out, just enough to reach the heater core. Unbolt the core and pull it out. Revers the process for installation.
Go to advanceautoparts.com and enter your vehicle's info. Search for heater core and it will show you a pic. Mike
Drain the radiator. Disconnect the heater hoses. Remove the screws holding the heater core case and remove the whole assembly. Remove the core and install the new on in the case. Reverse the procedure to finish the installation.
Drain the radiator. Remove the heater hoses from the fittings at the firewall under the hood. Loosen the dash, and pull it out enough to reach under to the heater core. unbolt the core, remove, then replace. Reverse the procedures to finish the installation.
If they disconnected the heater core, it means it was leaking. Get a manual on your car from the parts store and it will show you how to replace the heater core also
help me preplace heater core in 92 Chevy van g20