Heater Core Replacement

Winter is here and I have never been satisfied with the heat in my 69 Mustang. I decided since it was winter and I still want to drive the car it was finally time to figure out what was going on. I made some interesting discoveries. First let me review how most cars heaters I have worked on function. There is a slider in the drivers compartment which opens a valve on feeding heated water from the radiator to a heater coil under the dash. I have replaced a few heater coils and they are normally a bear to replace as they are so buried in the dash. The cause for replacement is normally a leaking heater core dripping water into the passenger compartment. In my case it was not dripping I was just not getting any hot air, or not a lot any way.  FYI my car has heat only, no AC.

To diagnose the heater problem I decided to back flush the heater core. I had done the entire cooling system a few years ago but this time I just did the heater core itself. I did this on my 76 Mustang II (bearly deserved the name Mustang) and it fixed the problem until I got rid of the car a few years later. I got out the garden hose and removed the hoses feeding the heater at the water pump. FYI you don’t have to totally drain the radiator to do this you just need to get the level lower than hoses your going to disconnect. If you catch the fluid in a clean bucket and you can reuse it. I put one end of the garden hose on the end of the heater hose and made sure the free hose was not going to spray me when water came out (would have been funny though). I simply wrapped a rag around the junction and held it with my hand to connect them. Turned on the water and sure enough water poured out of the heater core that was a deep rust color. I let it flow until it was clear then stopped. I reversed the water flow direction and more rust water came out. I continued this process until it flowed clear. The heater core did not seem clogged as the water flowed freely so I had done all I could. As I said this exact process fixed another car had. Being hopeful I put the car back together and gave the heat a try…it was better but not good…only warm. It was time for surgery.

I took the dash apart and got access to the heater core. When I put my hand on the bottom of the core with the engine running it was cool, the top of the coil was hot..I can only guess that the bottom of the core was clogged with sediment from over forty years of use and my only option was to replace it..I did look to see if the water control valve was bad … and this is where the interesting part is. The old Mustangs do not have one. The levers in the dash simply open a door to control the air flow over the coil. In other words the coil always has a full flow of water, interesting.  So that was it, time to replace the core.

Old Heater Core on the right. New one looked exactly correct and was a perfect fit.

I found a heater core online at Advanced Automotive for $40 with 20 percent off but did not want to wait for it to ship. I talked to the nice man at the local store and he worked some computer magic so I picked it up the next day at his store for $32, nice.

New heater core in hand I set off to work. As always when I start tearing into things I find other things I need to fix. This case was no different. Unrelated to the heater core I decided it was time to fix a bad alternator gauge that had bugged me for years, but that is another post. Once I had the old core out I saw the metal duct work was significantly rusted so I addressed that by hitting it with a metal brush and then putting a good coat of Rustolium undercoating on it. I also noticed that the door connected to the cool-warm slider was not closing. This would explain why I felt hot air on my ankles in the middle of summer. I adjusted the linkage so that it seated properly which should resolve the issue.

Heater core removal was fairly straight forward. Remove top of dash, front console panel over glove box, open glove box to give yourself more room, remove heater hoses and clamps and remove the tabs around the perimeter of the heater box. Also you have to pull off the round seal around the duct work from the air intake to the heater box. This is a bit difficult but do-able.  Once the tabs are removed from the box you can open it enough to remove the core.

Heater box. Here you can see the tabs that hold it closed. You have to remove them from the top, drivers side and bottom of the box to get it open. The duct tape is over the cowl vent I have yet to replace…that is a future project.
Old heater core in the heater box with the box opened, The glove box is at the bottom of the picture.
Rusty Heater Core compartment before I treated it.

Installation is the simple reverse of the removal process. On other cars once the new core is in they tell you to get the engine up to temp and open the heater to full…really does not matter on these cars as there is no heater valve for the water. Just start it and fill it…let it heat up to make sure all the air bubbles get out.

Heater box after treating for rust.
New heater core in place

The result of my hard work is that now I have heat.  I will wait for a nice cold day to see how well it works but it was indeed hot when I tested it in garage. Of course no good deed goes unpunished and now that I have the dash open I am going to fix the broken alternator gauge, I found the linkage that switches from defrost to heat was broken, Need to rewire dash speaker….and who knows…..the fun never stops :^).

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