This is rather involved and you need guidance and pictures. Get a manual on your car from the parts store or from AUTOBOOKSONLINE.COM
chiltons auto repair manual
You may want to go to a car parts store, and get a repair manual for your car. They cost about $16.00 Or, go to a Public Library.
Disconnect the negative battery cable and drain the cooling system.Disconnect the heater hoses from the heater core inlet and outlet connections in the engine compartment.Allowing residual coolant from the heater core to drain. Cap the openings.Working inside the vehicle, remove the lower instrument panel sound insulator.Remove the heater floor outlet duct screws or clips and remove the duct.Remove the heater core cover by removing the attaching screws and clips.Remove the heater core cover.Remove the heater core retainers and remove the heater core.
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Drain the water from your 1990 Chevy Corsica cooling system. Remove the heater core water supply hoses. Remove the heater core retaining bolts. Reverse the process to install the new heater core.
May have to take it to a certified speedometer repair shop
go to auto parts store and purchase a haynes repair manual
I found it in my repair manual. It is about 26 to 32 psi.
Remove the water supply lines from your 1990 Ford E3 50 heater core. Remove the heater core retaining bolts. Reverse the process to install your new heater core.
comes out inside . remove glovebox and go in that way . about one hour .
You pretty much have to remove the entire heater box under the dash on the passenger side
In order to change the heater core on an 90 ls400 you have to remove the entire dash pad and cross bar to be able to remove the heater core box. This is a job for and experienced LEXUS technician www.lextechautomotivellc.com