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84-93 Fox Heater Core Fix

Step 1: Replace 84-93 Fox Heater

Replace 84-93 Fox Heater

Fox Body Mustang (1984-1993) Heater Core Replacement Documentation
The following documentation is a report of my experiences in the world of 5.0 heater cores. Specifically, it included the history of my experiences and most importantly, how to change that heater core WITHOUT discharging your air conditioning, dispelling much false information out there on the subject! This is especially important these days with the astronomical prices of R12 and R12 system service. This document covers the 1985 GT 5.0 and the 1990 LX 5.0 models. Keep in mind that your model may be very similar if not exactly the same. A good amount of information contained in this document applies to the 1982-1993 model year 5.0 Mustangs.

Have you experienced the 'Green Pool of Death' or have you succumbed to constant fog on the right hand side of the windshield? If so your car probably has a ruptured heater core. I'm sure that you have heard all of the horror stories about changing this little headache out. I was quoted from a local Ford dealer that he would happily charge about $700 for the job. If this doesn't sound like much money to you, then stop reading this now, take your car down to the local Ford dealer and lay the headaches on them! Well, as for me, I'm very reluctant to part with that $700, so I'll do it myself, thank you.

I have done about six heater cores in Fox-3 Mustangs over the past eight years. So I consider myself somewhat of an expert on the subject.
My experience in late model Mustang heater core replacement started on my 1985 GT 5 speed back in about 1990 when I observed the green pool of death and a continuous foggy haze on the right hand side of the windshield.

So I began to attack the heater core project just like everyone had recommended, including removing the entire dashboard, and discharging the air conditioning system to remove the evaporator case from the firewall. And after much perspiration persistence, I installed the last screw in the glove box and fired the engine. The heater worked great, and no leaks! Sadly enough, this was not the end of the story.

I did this horrible job four more times. Each core only lasting somewhere between six weeks to a year. It didn't seem to matter if I used an off brand core or got one directly from a dealership. Why were these failing? Ahhh, what I needed was the police special service part heater core flow restrictor. So I made one and installed it. It was an aluminum standoff with a 0.25" hole drilled through the center of it. More details on that later.
The cores still failed.

While I installed the last core in that car, I made a very important observation. When I was closing the heater core cover with the two self-tapping screws, I heard a sound, like that of thin metal wrinkling. At this point I began to theorize why these heater cores fail.

There are two reasons that heater cores fail in Fox-3 Mustangs:
The first reason is normal wear and tear associated with thermal cycling. You can expect a new, factory installed (not dealer!) core to last between 5 and 6 years before it will start seeping coolant. This is given a 5.0L engine and the normal hard driving/abuses that this class of car is put through.
The second reason that heater cores fail is purely mechanical. At the factory, the cores are installed in the cases with a black gooey sealant called dumb gum. Over the years, the sealant gets hard. Not hard like a brick, but hard enough that when you install a new core in the same location as the old core, the sealant will not conform to the slightly different shape of the new core.
I'm not saying that the cores are dimensionally different, but the fins that make up the water passages, being in a zigzag pattern, are invariably in a slightly offset position, requiring that the sealant be displaced in a different pattern.

What I recommend is that it is ESSENTIAL to remove ALL of the black sealant BEFORE installing the new core! Unless of course, you enjoy this job and want to do it again. If you must use a sealant when re-assembling the heater core cover,
I recommend Dow Corning RTV 3140. RTV 3140 is a silicone coating used in the electronics & aerospace industries because it is very lightweight and flowable. 3140's flowability will avoid any distortion or flexing of the new core when the cover is tightened,


The most recent heater core job that I did was in my '90 LX 5.0 5 speed when it hit 66K miles last February. I did the whole job with the A/C completely connected!
The trick to it is to get everything loosened up, and REMOVE the bracket off of the A/C accumulator (engine_c.jpg ). This will allow enough freedom to get the evaporator case away from the firewall about one inch. It's very difficult and a royal pain, but you'll need to remove the two 5/16 self-tapping screws that hold the core cover in place. I used an small ignition wrench to remove the screws.
With everything loose and the dash propped up (drvrs_v.jpg ) and away from the windshield, you can open the core cover and remove the old core
(removal.jpg ). Removing the old core without damaging it is a good test to determine if you can install the new core without damaging it, so keep that in mind. It is crucial in this procedure to prop the core cover and the dash up out of the way so you can work (detail_1.jpg ). I used a large socket and a 1/2 drive extension (detail_2.jpg ) to hold the cover open (dashview.jpg ) to extract the old core and install the new one without tweaking it (very important.) Before installing the new core, I painstakingly cleaned, with surgical precision, the old black goo sealant that seated the old core (install.jpg ).
By far, the most difficult task of the entire job is getting the new core's tubes lined up with the gasket and holes in the firewall. On my car this step took the longest of any single step in the entire job. But don't despair, get a helper and with one person maneuvering the case on the inside and one person on the outside steering the lines to their propper place, you will get it, it just takes time!

How do you make the flow restrictors? I made them from 5/8 OD aluminum stand-offs drilled to 0.25" ID (restrict.jpg ). The heater seems to be effective but the motor has to be good and hot for the heater to be _really_ hot. I can live with this, but if I were to do it again, I might consider a 3/8 ID. After about 20 minutes of normal driving, I observed air temperature of 157F. This is with a 192F thermostat.
When installing the heater core restrictor, make sure that it goes in the input or pressure side. The input/pressure side is the top metal tube that runs along the intake and the larger of the two pipes on the heater core.
With improved clearance in the core compartment and the restrictor plug that I machined, installed, I hope to see at least 66K miles out of this one.
Here are the images documenting the heater core procedure on my '90 LX.
removal.jpg Removing the old core. dashview.jpg Dash view, foam bricks used to support the dash under red rag. 1/2" extension bar to hold dash up and away from the working area. drvrs_v.jpg Driver's side view. Instrument cluster removal may not be necessary for this job. My speedo cable popped out and I had to reconnect it. engine_c.jpg Engine compartment view. restrict.jpg View of my homemade aluminum coolant flow restrictors. 1/4"ID, 5/8" O.D. install.jpg Heater core cover supported with extension bar on left and screwdriver on right. I found that I needed something bigger than the screwdriver to install the new core. Only aft side cover screws removed. detail_1.jpg Detail close up of inside cover. detail_2.jpg Detail close up of cover supports. Please note: The A/C system was left fully charged and no plastic or other material was cut or removed in the process. The whole job took about 9 hours, including some other minor unrelated tasks.

Dash View

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Cover Supports

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Inside Cover

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Driver's Side View

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Engine Compartment

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Heater Core Removal

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Heater core install

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