Replacing the Heater Core on a 1964 Buick Wildcat with A/C
This should also be identical on a 1964 Buick Electra and LeSabre. Non A/C cars will be completely different.

Replacing a heater core is one of those jobs that immediately send most people to the mechanic. How do you know your heater core needs replacing? Usually a puddle of antifreeze in the passenger side floorboard soaking the carpet, or if you are lucky, a puddle of antifreeze under the car away from the motor. I was lucky in that my heater core manifested the latter failure method, leaking around the lower heater hose opening.

If you open your factory service manual for advice, it will read "disassembly will be obvious upon inspection." Yeah, maybe not. My father, who previously owned the car, had replaced the heater core last in the 1980s, so 20+ some odd years on a heater core is a pretty long time. I'm surprised it didn't leak earlier, like maybe at a convenient time like when I already had the A/C system torn apart.

To begin, source your replacement heater core. These are not unobtainable, but I don't think any parts store will have these on the shelf for immediate pickup due to their narrow application. After searching for the best price, I purchased mine from Advance Auto. Interestingly enough, they have two heater cores with the same part number, 399062, and the same lifetime warranty. They had a "Ready-Aire" for around $66, and A "Ready-Rad" for around $46. Both "Ready-" companies are owned by Proliance. I chose the Ready-Rad and paid $50.13 out the door after tax. It came the next day from their warehouse, and the only idea of the price difference I could find is the Ready-Rad is made in Mexico. I don't know where the Ready-Aire product would have been made. Buy a gallon of antifreeze while you are at it, and some new heater hoses and clamps if yours need replacing.

Once you have your parts, get your digital camera ready to take pictures at each point of disassembly. Also, I recommend getting a big piece of cardboard you can poke fasteners through so you can note which fastener goes where. You will probably remove close to 50 fasteners on this project.

Start by removing your glovebox door, glovebox (you will have to remove your light), center A/C vent, center A/C vent plenum, radio faceplate, then radio. If your radio is screwed in at the bottom you will have to remove your ashtray, ashtray light, and cigarette lighter connection. Remove your radio speaker at this time. Your dash should start to resemble this:

Also, look under the dash and find where your dash vent hoses connect to the distribution plenum in the center of the underside of the dash. Pull each hose off this plenum.

Once that is complete, I recommend you move to unhooking everything related the the heater box inside the car. If you look down through your glovebox at the heater box, you will see this group of wires and hoses:

It has been rumored some cars have a plastic cover over this, so if that is the case remove it. What all of this cable and vacuum hose does it covered on my A/C diagrams page. What you need to do is take a picture of all of this on your car, then start disconnect all the wiring and vacuum hoses. If you find your hoses are brittle and crack, and auto parts store can sell you replacement bulk hose and you can paint a stripe down the side of it to match the factory color with spray paint. You also need to disconnect the yellow control cable (shown below already disconnected for clarity):
 
To do so, first remove the star retaining washer on the shaft it is pushed down on. Next, remove the nut holding the mounting tang. The cable should then slip free. You can see my mounting tang broke off my cable.

Once you disconnect everything, the area should look like this:


Next, remove the gas pedal, center heater distribution outlet, and carpet trim ring. In the picture below, you can see the accelerator pedal is mounted on two metal rounded studs. The pedal can simply be grasped and moved back and forth to remove it.

Now, remove the air distribution plenum. This was the box each of your dash vents was connected to. To do so, you first have to move the large black rubber band that connects the plenum to the heater core box. Push it forward or back, but just get it so it is not connecting the two. Then, remove the two screws that hold it to the bottom of the dash. Provided you took out the center vent plenum already like you should have, the plenum will slip right down. Below see the below and after of removing the plenum.
 
Remove the two heat distribution outlets that are above and to the back the metal main heat duct on the driver's side.

Next, it is time to remove the other control cables. On the right side of the heater distribution plenum, the cable that controls the amount of heat to the rear of the car needs to be removed. Once again, remove the retaining washer and then the nut that holds the tang on. Next, there are two cables that are attached to the heater control box. Notice they fit one tang into the other. It is important to remember how they are mounted together for reassembly. One comes from the heater control knobs to adjust the air doors in the heater box for temperature, and one goes out to the heater control valve under the hood to regulate the amount of hot water flowing through the heater core. Also, there will be two vacuum hoses going to a valve next to these cables. If you trace these two vacuum hoses, one of them should already be unhooked and one should go to a tee. I've found the back hose impossible to pull of when the box is mounted, so I recommend disconnecting it at the tee.


Now move to the right side of the heater box, and look up into the dash area. The yellow striped vacuum hose needs to be removed from the vacuum actuator that controls if heat comes out of the side vents, and the yellow tanged control cable needs to be removed from the defrost control. Here you can see that my cable has already broken and it held on by cable ties.

Now it is time to remove the metal lower distribution duct that channels warm out to the back of the car. To do so, pull the carpet back until the see the top of the black retainer. Remove the two screws, one from each side, from the retainer, and put it aside. Next, remove the two screws at the top of the metal plenum that secure it to the fiberglass heater box. Then, remove the screws on each side of the metal duct that secure it to the body. Pay attention to how the metal distribution duct is layered in with the plastic tubes that go under the seat.


Once you rock the distribution plenum out of the way, you should end up with about the same layout as below. The black and silver box on the floor in the picture is an FM modulator.

By this time you should end up with a pile of things you removed that looks something like this:

Now is a very good time to take a break or wrap for the day. Next up is getting under the hood and the dash to remove fasteners, and a wrestling match with a fiberglass box.

Next, go under the hood and loosen the hose clamps for the two heater core connections, and remove the hoses. Do this before you remove the mounting screws to the heater box, so you have good leverage. Below I have outlined the heater hose path for your convenience. Also shown are the top and bottom heater hose locations.



As a word of caution, be careful around your vacuum diaphragm. It will be brittle with age. I knocked in to mine and the plastic teat broke off. I expoxied in a vacuum hose splice kit to replace the broken part, as seen below. 


Next, undo the bolts that go from underhood into the passenger compartment. If my memory serves me correct, there are two or three. You can spot these by looking in the passenger compartment and looking for clips, such as shown in the first photo. The second photo shows the location of one of the bolts.

Next, come back in the passenger compartment and remove the bolts holding the heater box to the firewall. You will find one to the right, and one within the output plenum, and probably some more I have forgot about. (You should have already removed the heater control cable circled in picture 2).
 

 

At this point, everything should be undone from the passenger compartment fiberglass box and it should not be attached via any screws to the underhood fiberglass box. Take a moment to double check nothing is connected. Now, at this point, you might look up and realize there are screws all the way around the box and think you can just under the little screws, swap the heater core, and not have to wrestle with the box. Well, I tried this, and it is impossible to remove the heater core with the fiberglass box in the car because of how they have the heater core strapped in, with the strap screws facing the firewall.

Now take a real good grip of your fiberglass box, and wrestle it until it is detached from the firewall and maneuver it down to the passenger side footwell.

Next, remove the box from the car. This is what the inside of you car will now look like. You can now see the A/C evaporator.

Here is the heater core's fiberglass box out of the car. I have already removed the defroster duct at the top of the case and evident in picture two. The white sealant you see in the picture is caulk (improperly) used during the last heater core replacement). Remove the screws holding the two halves of the box together and then separate the box.

 

In the first picture, you can see the heater core and where it failed at the bottom. Notice it is held in with red metal straps. As shown in picture two, remove the two straps with a flathead screwdriver. The straps are shown in picture three. I sanded and repainted mine for added protection.



Here you see the heater core as removed. In the second picture, you can see the old core on the left and the new core on the right. The old core failed at the bottom.
 

I had another bad heater core on hand from a parts car, so for comparison you can see that these will not fail all the same. On the parts car is failed at the top, while it failed on the bottom in my car.



Strap the new heater core in and replace the halves of the box together. I used a very fine line of black automotive sealant between the two.

Once the two halves of the box together, now is a good time to connect some heater hoses to the max cooling vacuum valve. This is a very difficult valve to get to later, so make sure it works as expected at this point. I couldn't get this valve to release and later found that one hose should curl under the metal actuator to provide tension for the valve to release.



Since the inside of my firewall showed some surface rust, I took measures to protect the car and sanded the rust and painted on a rust converter. These are the before, during, and after photos. Since all the area is hidden from view, I did not paint over the rust converter, I just left it it's natural black.


Once the box was re-assembled, I put a bead of automotive sealant around where the air handling part meets the firewall, and hoisted it into place.

Once you wrestle the box somewhat into place, add the defroster duct. Notice how it is in the second picture below. The correct orientation is for the notched out area to be the right. Putting this in the wrong way will result in it breaking (notice the JB Weld on mine).

 

 Next, hook everything back up the way you took it out. Make sure you don't kink and control cables and be sure to run the engine with the radiator cap off for a few minutes to purge any air out of the system.