How to Replace the Heater Core on a 1985 Oldsmobile Cutlass

Your 1985 Oldsmobile Cutlass has probably logged plenty of miles and might even have a few stories to tell. If the passenger compartment is no longer getting warm in winter, or you find coolant leaking from under the passenger dashboard, chances are you need to replace the heater core. The heater core is a smaller version of your radiator and it works the same way. Air blown across the core's fins draws heat energy from the hot engine coolant passing through it. If the core needs replacing, do it yourself to save money.

Tools Used: Socket wrench, Screwdriver set, Pliers

Replace Heater Core


Disconnect the negative battery cable with the socket wrench or box wrench.

Drain the cooling system by opening the radiator drain on the bottom of the radiator with your fingers or pliers. Open the drains on the engine block just above the oil pan with a wrench and drain all coolant from the engine block. Collect all of the coolant in a tub or pail for re-use. You only need to replace the coolant if it appears cloudy and contaminated. Close the three radiator drains securely.

Locate the heater core inlet and outlet hoses on the passenger side fire wall. Disconnect the two hoses using a screwdriver or pliers. Drain coolant from the heater core and hoses. Close off the inlet and outlet pipes on the heater core using plastic and/or rubber bands to keep dirt and contaminants out.

Look under the dashboard on the passenger side and locate the lower instrument panel sound insulator. This is a plastic panel just below the glove box. Remove the panel by loosening the clips holding it to the dashboard.

Remove the heater duct by undoing the clips and screws that hold it in place. Remove the heater core cover by undoing its clips. You might need pliers to undo some of the metal clips. Take off the heater core mounting bolts and retaining straps with a socket wrench. Remove the heater core.


Put your new or rebuilt heater core in its place under the dash. Put the retaining straps in place and screw in the mounting bolts with a socket wrench.

Put the heater core cover in place and fasten the clips and screws. Install the outlet duct with clips and screws. Install the lower instrument panel sound insulator clips and screws.

Go to the engine compartment and remove the plastic covers from the hose ends on the heater core. Re-attach the inlet and outlet hoses.

Open the radiator cap and pour coolant back into the radiator until the fluid reaches the bottom of the radiator filler neck. Pour coolant into the overflow tank to the "FULL" mark.

Connect the negative battery cable with a socket wrench or box wrench. Start the engine with the radiator cap still off and let it idle.

Look in the radiator. You should see the coolant level has dropped somewhat with the engine running. Pour more coolant into the radiator until it reaches the base of the filler neck. Put the radiator cap back on with the arrow pointing to the overflow tank. Turn off the engine and check all the hoses, connections and drains for leaks.

Tips & Warnings

GM recommends a 50/50 coolant/water mixture, but you can increase the amount of coolant if you live in colder climates and require greater freeze protection.

Only work on your heater core and other cooling system components while the engine is cool to reduce the risk of burns.

Remember that coolant is highly toxic and tastes sweet, so pets and wild animals are drawn to it. Make sure coolant is stored properly and any spills are cleaned up.

Never use more than 50% water in your coolant because your engine could overheat while the temperature gauge shows "normal".

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