If you've owned an early Mustang for very long, you've probably already dealt with a leaking heater core. I've had to replace the cores in both of my Mustangs. But if you've never faced it, here is a step by step guide to doing it yourself. It isn't an impossible job, just not one of the most fun jobs. An early indication of leaking heater core is excess humidity inside the car. You turn your defroster on and the windshield fogs up quickly. Feel the fog and its not just humidity. You might also smell something sort of sweet. Feel the carpet on the passenger side of the cockpit, under the core. If you feel dampness on the carpet, bad news the heater core is leaking.

A quick fix for an early Mustang is to bypass the heater hoses. You'll need a short piece of heater hose to connect the outlet on the water pump and the block and a couple of hose clamps. You'll also need to connect the two long heater hoses running to the heater core in the cockpit. Or else drain these two hoses. I saw a valve that connects the in hose with a turn off valve that lets you close the coolant from going to the core. This is not a concours fix, only a temporary fix. It will stop the coolant from going to the core and into the car.

The only way to fix a leaking heater core is to replace it. Most parts vendors carry replacement cores. I'd recommend you also order a couple of new heater hoses. Now is a good time to replace them as well. You don't want to have to do this job again later when you discover one of the hoses leaking. The hoses connect to the rear of the core. You have to remove the core to replace the hoses. This would also be a good time to replace those aftermarket hose clamps with NOS ones, too.

The first thing you should do when you are ready to start the job is drain some coolant from the radiator. You don't have to empty it, just drop the level enough to not have coolant spilling out on top of the engine and floor when you disconnect the hoses from the block. There is a drain valve on the bottom of the radiator. Antifreeze is toxic to children and other living things so be aware of where the coolant goes. Use an oil drain pan or something to catch the fluid under the car. If the coolant is in good shape you can reuse it when you are done with this job. Other wise dispose of it properly. Down the drain is not proper. Do not leave it out where animals can get to it. They will drink it and they will die. There is a new non-toxic antifreeze solution available today.

Next I removed the glove box to give me more room to work. Remove three screws on the bottom of the dash supporting the glove box. 

The next thing you need to do is remove the cover from the heater box. It has clips around the raised seam. Using a flashlight, notice the clips in the right side, top, bottom and left side. There is a special tool that spreads these to make removal and installation easier. A flat head screw driver will also do the trick. Put your screwdriver under the lip of the first clip and pry up carefully. They will pop off. Remove all the clips and place them in a container. ( I use the bottoms of milk/juice cartons to hold my parts.) There is a control wire on the top of the box attached to one of the hinges. It will lift off with some assistance from that screwdriver.. On the right side of the cover is a round foam piece that encircles the cover and the back of the box. Carefully pry it up from the front of the cover. Note how this is assembled because you'll have to put it back later.

You are going to need some light to see what you are doing. This is one of those stand on your head jobs. If you use a trouble light be careful that you don't burn yourself on it. There isn't a lot of room and those lights get damn hot. You can get a nasty deep burn from a trouble light. I know.

What kind of shape is the cover in? Replacement covers are available if its in bad shape. The foam rubber inside will probably look bad from age. If you are going to reuse the cover, vacuum the leaves, rust and dirt out of the cover.

Removing the cover is much easier if the heater hoses are loosed from the block. You have to pull the core out to remove the heater hoses from the left side. Loosen the clamps at the block and pull the hoses off. (You did drain some coolant from the block. If you didn't you're going to have a mess on the engine and floor.)  Pull the core out. You'll be pulling the heater hoses through the firewall as you do this. When the core it out far enough for you to get to the hose ends, loosen the clamps there and pull the hoses off. Hopefully there is no more fluid in the hoses. Be prepared to catch it if there is rather than spill it on the carpet.

Pull the old hoses completely out either from inside or outside the car. Feed the new ones back through the two holes in the firewall. NOS hoses have a red line down them. I would position the line so it is up and viewable. Time to put the new hoses on the new core. Crawl back inside the car and put the hose on the rear of the core. Slip the clamps on first. Push the hoses past the raised part of the core ends. Slip the clamps over the raised part and tighten the clamps as tight as you can get them. Don't want any leaks here. Push the heater core back into the box.

Now would be a good time to test for leaks. Push the other ends of the heater hoses onto the water pump and the block. Of course put the clamps on the hoses first and tighten them.  Refill the radiator. Feeling lucky yet? Start the engine and let it warm up at least until the hoses are warm or the radiator is warm. The thermostat needs to open to allow the hot engine coolant into the heater hoses. Turn the car off. Back inside the car check the core for leaks. Feel around behind the core at the hoses to make sure you got the hoses on tightly. Watch the core because the hot coolant will make the core real warm. If you find leaks, now is the time to do what has to be done. Hopefully the core doesn't leak. That would be not good but it is also not likely. If the hoses are leaking, you did something wrong, probably with the clamps. Re-check your work.

If you have no leaks, its time to put the cover back on the box. The cover fits one way. There is a small cut out on the bottom of the cover to fit around the inside door post. Remember the round foam piece on the right side? I worked both sides of it around the round lip on the cover and pushed the front up out of the way. Gradually work the cover in place. Use a small screw driver to pry the foam over the front of the round piece on the cover. It should slip over. Check the ridge all around the cover to make sure it is lined up. Find your clips and put them one of a time. I did the left side first, then a top one, next one of the bottom ones. Some of my clips clicked right on with little effort using the flat end screwdriver. I put the clip in place on the back of the box and pushed down on the front. I had a few that wouldn't snap on. You can push and push but what I did was take a couple of pliers and very carefully spread those open a little more. Be careful not to break on or spread it too far. Won't take much. Then snap them on with the pliers. (If you had one of the clip tools you wouldn't be doing this part with pliers and a screw driver.)  Congratulations, you just replaced your heater core.