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Fox Body Mustang Heater Core Replacement | 79-93

Created by Jeff Jimenez
Last Updated 9/20/2021

Is the heater core in your Fox Body Mustang leaking? Check our LMR's how to replace and install tech article to get yours fixed today!

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Fox Body Mustang Heater Core Replacement | 79-93 - Fox Body Mustang Heater Core Replacement | 79-93

Replacing the heater core on your Mustang is inevitable! At some time or another during ownership, you will have to deal with the dreaded Heater Core Replacement. The most common tell-tell signs that your Mustang Heater Core needs to be replaced; are a strong odor of Antifreeze coming through the vents, continuously fogged up windshield, and wet carpet on the passenger side under the heater box. Most people just loop their Mustang Heater Core off so it doesn't leak anymore, but what happens when winter comes or you have an unexpected cold front come through? You don't have a Heater! To replace your Mustang Heater Core, most shops charge anywhere from $250-$400. I've even heard of dealerships (or stealerships, whichever you prefer) charging $600-$800 for the same job! I don't know about you, but I would rather spend a weekend and less than $100 to fix my Mustang Heater Core myself and keep that extra pocket change to pay the bills.

I recently bought a 92 Fox Mustang LX 5.0L for my daily commute to work and it had a bad Heater Core. I decided to replace it myself and let everyone see what all is involved in replacing a Mustang Heater Core so they could attempt it too if they felt so inclined. I used our Heater Core Kit that includes the heater core, heater hoses, and clamps.

The following instructions are for 87-93 Mustang, but 79-86 Mustang is similar with the main difference being fastener locations on the dash & console.

This is a pretty big job that requires you to pull the dash from the car so the first order of business is to disassemble the interior components. I started with the removal of the center console.

At the rear of the console are Access Plugs on both sides of the console. You will need to remove them to get access to the armrest mounting bolts.

fox body heater core removal

Once these are loose, the armrest can be pulled out revealing the 2 Console Top Panel Screws. Next you need to take the Shifter Bezel loose. Unscrew the shift knob so the boot/bezel can slip over the top of the handle. The bezel is held into place with 4 spring clips at each corner. Pull upward on the bezel to disengage the retainer clips.

fox body heater core removal

After the bezel is out of the way the 2 front screws are exposed. Once these are out, the console top panel will be loose but not free yet. There are wiring harnesses going to the mirror switch and cigarette lighter. Reach under the top panel to take disconnect the harness. With these disconnected, the top panel will be free to move out of the way. Next, you need to remove the screws that hold the console to the dash. They are located behind the lower dash panel on the driver’s side and behind the glove box door on the passenger side. To gain access you need to take the lower dash panel loose. Remove the lower bolts and pull on the panel to release the spring clips that fasten it to the metal retainer plate.

fox body heater core removal

With the panel removed, you can see the metal retaining plate and the bolts holding it on. Remove these to expose the rest of the dash as well as the steering column. Now that these are removed, you can see the screws that hold the console to the dash. Remove these screws as well as the trunk/hatch release button so the console is free.

fox body heater core removal

With the console loose, now you need to remove the A/C Control Bezel and radio. The A/C Control bezel just snaps in place so very little pressure is needed to free this item. Radios are installed in many different ways so there will probably be steps specific to your application that I can't cover. Once these are removed, the console is ready to come out. (Note: you might have to remove your shifter handle to make clearance for the console to come out.) With the Console out of the way you can start taking the dash loose. Start by taking the instrument bezel/cluster loose from the dash. There are 2 screws up by the windshield and one located by where the console was mounted to the dash. "pic was taken looking through the windshield at the top of the dash".

fox body heater core removal

With these taken out the instrument bezel is loose with the exception of the flasher/defrost switch and the headlight/foglight switch. To release these you need to depress the 2 retaining tabs to allow the switches to slide out.

fox body heater core removal

After the switch is loose, then you need to disconnect the wiring harness from the back and you can pull the bezel from the dash. With the bezel out of the way, you can see the 4 screws that hold the instrument cluster in place. Take those loose so you can reach behind the cluster and take loose the Speedometer cable as well as the 2 wiring plugs "1 by the tach and 1 by the speedo" that plug into the back of the gauges. To release these you need to press in the 2 tabs on the bulkhead and the plug will pop loose. To release the speedo cable you need to press in a single tab on the side and pull so it will disengage from the speedometer. (Note: there is not much room to pull the cluster away so space will be tight to accomplish this.)

fox body heater core removal

Now that the cluster is out of the way you can loosen the 4 retaining nuts that hold the steering column in place. They don't need to be taken completely off just undone enough to drop the column an inch or so. With the column dropped down a little you can access the support nut that secures the dash to the pedal assembly. Loosen this nut and back it off about 1/4". This will allow the dash to come undone once the rest of the fasteners are removed. Next, you want to take loose the 2 lower dash bolts located by the doors just under the lip of the dash.

fox body heater core removal

Once those are loose the only thing holding the dash in place is the row of screws just under the windshield.

Remove the defrost vent cover by inserting a common screwdriver between the layers and twisting it to create a place to stick your finger in. Pull gently on it to release the clips holding it in place. You also need to pull the speaker grilles off the dash. There is a single screw on the side that needs to be taken loose and then the retainers pop loose when you pull on them. Now you should be able to see all the screws running along the length of the dash holding it to the body. Take these loose and the dash will be able to be pulled back. You don't want to take it completely out, just move it enough to where you have access to the backside.

fox body heater core removal

Now that the dash is loose you need to move to the under hood of the car and disconnect everything from the firewall.

Start by taking loose the Nuts holding the A/C Dryer to the Firewall, My A/C system was NOT charged so the next thing I did was to move the dryer out of the way. * * * DO NOT ATTEMPT TO DO THIS IF YOUR A/C SYSTEM IS CHARGED AND FUNCTIONAL THESE LINES ARE UNDER PRESSURE.* * * If your system is charged, you can either #1 have a shop evacuate and recharge your system or #2 you can attempt to leave the lines hooked up. If you choose option #2 you must be very careful when pulling the heater box from the dash as you could damage major components of your A/C system. Since mine wasn't charged I pulled loose the dryer and disconnected the A/C line with the special spring-lock disconnect tools and moved it out of the way to give me more room to work.

fox body heater core removal

Now that that was out of the way, I had room to disconnect the heater hoses that connected the heater core to the Heater tube assembly on the motor. There will be some residual coolant in these lines so be prepared for a small amount to pour out. With the lines lose you will need to take the 2 large diameter nuts off on the firewall that hold the heater box secure to the firewall.

Once those are loose, you are finished under the hood and need to go back inside the car. There is 1 final bolt that is holding the heater box in place. It is located on the bottom corner of the heater box beside the transmission tunnel. Once that is loose the heater box can be moved. Pull back on the heater box assembly to expose the plastic cover the heater core is located under. Remove the 4 small bolts that are holding it in place.

fox body heater core removal

With the bolts removed you can pull the access cover off to expose the heater core. It is just sitting in place; the only thing you have to do is pull the tubes out of the rubber insulation that seals off the opening in the firewall. Be careful because there is residual coolant in here as well and it will spill onto your interior. Trust me, I know from experience.

With the old heater core gone you can clean up any debris and coolant that might be in the opening. We offer 2 different versions. The Aftermarket Core and the OEM FORD Heater Core unit. I used the aftermarket version. The new Heater core is slightly smaller and made of aluminum instead of copper. Supplied with the kit is a strip of foam with adhesive backing. It is designed to go around the perimeter of the core to hold it in place and prevent it from sliding around in the box. Install the new core in the heater box being careful not to tear the sealing foam that goes around the tubes.

fox body heater core removal

Reinstall the access cover to seal the box back up. Now comes the tedious task of putting everything back together again. It all goes back together just like it came apart just in reverse order. I recommend going back with new heater hoses just to give you peace of mind. The new Motorcraft hoses come with a coolant flow restrictor installed to reduce the chances of the new heater core blowing out the same way. If you choose to go on with your existing hoses and they don't have the Flow Restrictor we also have those available separately.

fox body heater core removal

Well, we hope this helped some people out. It is a pretty involved process and will take the majority of a weekend to do but it can save you some cash if you don't mind diving into projects like this. If not at least this gives you an idea of why shops charge what they do for a Fox Mustang Heater Core Replacement.

1979-93 Mustang Heater Cores

About the Video

Fox Body Mustang Heater Core Installation (79-93)

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Published on 2013-11-04
The heater cores in Fox Body Mustangs are infamous for leaking and causing poor performance of your heating system. We get a constant bararage of questions on how to replace 79-93 Mustang heater cores. We listened to the outcries from our fellow enthusiast and we created a step-by-step walkthrough video on how to replace your heater core. The installation does require you to have roughly a half to full day of time set aside to tackle the job; however, doing the job yourself can save you hundreds of dollars!!!

A bad or leaky heater core can cause steamed up windshields, strong coolant smell coming from the dash or even worse -- wet passenger side floor. Don't let a bad heater core ruin you carpet and cause you to drive around in the cold! Pick up a 5.0Resto replacement heater core today and watch our installation video! We have them available for both A/C and Non A/C Mustangs.
As you can imagine, latemodelrestoration.com employees have varying levels of projects going on in their spare time. And this one quite literally just came out of the weeds. It does run in drive now, but it is in dire need of a heater core, and we know you've been begging for that install video, so we're going to give it to you. As with any installation, you'll want to start by disconnecting your battery and with this, you need to remove your seats in your center console. You can check out our console and seat removal videos to show you exactly how to do those steps.

Now, let's get on with pulling the dash. Remove the four bolts that retain the lower dash cross brace and then remove the brace. Loosen the front scuff plate screw. Remove the kick panel pushpin, and remove the cake panel. Do the same on the other side. Pull out the cluster of wires on the passenger's side, and separate all the connectors.

Remove the two nuts that hold the hood release cable bracket to the steering column and then let the cable and bracket hang free. Loosen the two nuts closest to you on the steering column, and then remove the two nuts near the firewall completely. Unplug the airbag, cruise, keyboarding, and ignition switch connectors. Support the column and remove the two nuts closest to you keeping the columns supported lower enough to unplug the three multifunction switch connectors. You can now allow the column to hang freely.

Remove the top two cluster trim bezel screws. Remove the bottom right cluster bezel screw. Using a small screwdriver, release the retaining tabs on the headlight and hazard switches. Remove this switches from the bezel and unplug all the connectors. You can then remove the trim bezel.

Remove the four cluster retaining screws. Disconnect the left-hand cluster connector. Disconnect the speedometer cable by pressing down on the plastic sleeve and then sliding it off the speedo housing. Disconnect the right-hand cluster connector and remove the cluster from the car. Loosen the single nut retaining the dash frame to the pedal assembly. You do not need to fully remove this nut.

Remove the speaker grill retaining screw in the speaker grill on both sides. Remove the two glove box door screws and remove the glove box. Remove the defrost vent trim. Remove the five screws along the top of the dash. Remove the single bolt at each lower corner of the dash at the kick panel area and then carefully lower the dash.

Unplug the blower motor, vacuum connections, and blower resistor along with the radio amp if equipped. Over by the pedal assembly, unplug the brake switch connector. Rotate the dash out of the way carefully. Remove the single both at the base of the HVAC box. Loosen the two bolts on the HVAC box upper brackets. Remove your heater hoses at this point if they're still connected. Our were already bypassed.

Remove the two nuts that retain the accumulator bracket to the firewall. Loosen the accumulator bracket screw and slide the bracket all the way down the accumulator. Remove the two large diameter nuts retaining the HVAC box to the firewall. Disconnect the vacuum line. Back inside, fully remove the two upper bracket bolts you loosened earlier and pull the HVAC boxes far in as possible. This will expose the heater core cover lid.

Remove the two cover screws closest to you. Taking your time, trust me you'll need it, remove the two cover screws near the firewall. Lift up the cover and remove the heater core. This is where it was leaking at. Take your new heater core and wrap it with the supplied foam tape. Slide the new heater core into the HVAC box, and reinstall the cover lid and four screws. Believe me it's not any easier than taking them out.

With the HVAC box back in place, you can reconnect your heater hoses and tighten down the clamps. Be sure to check and top off your coolant level. Reverse the removal procedure of the HVAC box. To hang the dash, line up the locating tab in the center of the dash with the hole in the firewall and the single nut you loosened but didn't remove with the slot on the pedal assembly. With the dash back in place, reverse the removal steps for the dash.

You'll notice we completed this job without taking lose anything with the AC. Now, if you were to go and have your AC system evacuated, you could disconnect the accumulator and your liquid line from the evaporator and pull the box completely out giving yourself a little bit more room to work. Couple of other things, you're going to want to have a drain pan to catch any coolant and after you hook you heater hoses back up to your new heater core, you want to check your engine coolant and top it off.

Please feel free to leave a comment if this video was helpful for you, and if there's any other videos and you'd like to see from latemodelrestoration.com.