Heater Core Leak

1997ThunderbirdLXV6

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Alright, here it goes. I chose the thread title based on my best assumption - what else could it be?

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Sequence of Events

I was on my first longer trip since the transmission cooler installation: Richmond-Greensboro-Richmond, specifically the western route via Danville. My app was connected because I wanted to monitor transmission temperature, which topped out temporarily at 152°F but mostly settled in the range of 135-145°F; engine temperature was also slightly down at 199-203°F. For anyone unfamiliar with this route, it's mostly state routes with rolling hills; with cruise control set to 65 mph, the transmission unlocks the torque converter to maintain speed uphill, but it doesn't shift down to 3rd.

I hadn't checked coolant directly before the trip, but I checked it a few days prior, and all was normal.

On the way home, about 40 miles out, I suddenly got a Low Coolant light. I pulled over as soon as possible, still expected a false reading. Coolant was indeed about 3" above the bottom of the bottle. Fortunately I had a jug in the trunk; I didn't measure exactly, but it took at least a quart to bring it to between the COLD and HOT markings. Since I couldn't see any obvious leaks in the engine compartment, I proceeded to the passenger foot space, et voilà...

I drove home, and there was no additional Low Coolant light. When I got here, the level in the bottle had fallen at most a quarter of an inch.


Diagnosis

I suppose I should take a good look at the hoses going to the heater core to rule them out. I didn't have a good flashlight handy, it was raining, and I was just over it at that point.

Assuming the heater core is to blame, there are some oddities though:
  • Where is all the coolant? The amount of coolant on the carpet just doesn't seem to amount to what was missing from the reservoir. Is it somewhere in the ductwork?
  • Does it have anything to do with turning on the heat? This drive was the very first time this fall that I turned the heater on
  • Can you hear a heater core leak? Earlier during the drive, I suddenly noticed a whooshing sound. I couldn't put my finger on it and ultimately attributed it to the road surface (wet), then I turned up the music.
  • What did I do wrong? My coolant has been changed once by me; the previous coolant was clean, too. I use conventional green. I periodically replace just the coolant in the reservoir because it's easy, so why not. I don't have any added grounding on the heater core, but remember that Ford TSB that specifically advises against adding a ground. A while back, I checked for voltage in the coolant with a multimeter based on some online how-to, and it was fine.


 Repair

I know there's a tech article entitled "Dash removal and heater core change" by @XR7-4.6 , but I can't see the photos in it:
Screenshot_20250917_151848_Samsung Internet.jpg
Could that article be repaired?

Has anyone come up with an easier way of doing this? Like replacing the heater core by cutting a hole into the sheet metal under the backseat or something like that? Or...more seriously, by reaching through the airbag area, glovebox area, etc.?

What parts do I need other than the heater core itself, if any? For the heater core, any particular brand, type (copper?), etc.?

Did the leaking coolant potentially damage other components? OBD port, blend door, etc.?


 Mood

Look, I'm not happy. This job seems maximum tedious. This is the first car I ever did serious work on, so every job I do is my first time doing it.

But...I know I shouldn't complain. I drive a car I love driving, and I own it as a daily driver at a fraction of the cost of any modern car, even with the expense of an occasional repair.

Which leads me to my last (for now) question: given the work I have done till now, do you think I could complete this in one day? Or is that too optimistic?
 
Heater cores fail. It is almost 30 years old. This really is not a bad job. It looks big and scary. In reality, all it takes is some patience (that and another hand to help move the dash in and out of the car if you choose to do so). I had a writeup on sccoa sans pictures a loooong time ago that also went through it step by step.
 
Which leads me to my last (for now) question: given the work I have done till now, do you think I could complete this in one day? Or is that too optimistic?

Only time I replaced the heater core in a MN12 was when I pulled the dash to add the clutch pedal.

I would suggest planning two days if you can make that work. Take out the dash and get the new heater core in on day one. Put it back together the second day. You can get it done in one day, but IMO you want to take your time. Start rushing jobs like this and you might end up cracking a brittle aged interior piece, or find you have "extra" hardware when you are finished.

It's really not too bad it just takes forever and is tedious. Lifting the dash out of the car is a bit of work it's heavy iirc. If you are in good shape, it's possible to get the dash out yourself. You can probably get to the core by pulling the dash back a few feet and leaving it in the car. Probably easier that way.
 
It is really not too bad. Just watch this video and remember DO NOT PRY AGAINST WINDSHIELD!.

 
If it’s your first time doing it, I would plan for a weekend, not just a day. Pulling the dash really isn’t that bad on these cars. I have done enough of them that I can have the dash out in about 45 minutes. One tip from my experience, some people drop the steering column and pull the dash without it. While that does make the dash lighter, it is a lot of extra work. I just disconnect the shaft from the bottom of the column, and pull the whole dash with the column. It is heavier and harder to maneuver, but if you have an extra pair of hands for that one part, it saves a lot of time. Also another tip is the dashboard doesn’t have to come out of the car, just back away from the firewall. If you pull the front seats, you can push the dash back almost to the rear seat leaving plenty of room to pull the hvac box, and no need to try to carry the dash in and out of the car.
 
I know this day is coming for me, I feel your pain. Do you guys recommend replacing the AC piece while in there, it’s also 30 years old?
 
  • Where is all the coolant? The amount of coolant on the carpet just doesn't seem to amount to what was missing from the reservoir. Is it somewhere in the ductwork?

Before asking that question, I should have lifted the floor mat...

20250917_170735.jpg20250917_170742.jpg



Meanwhile, I'm confused by what I'm seeing on Rockauto. All these heater cores have nozzles pointing into opposite directions; the one on my car has both pointing toward the passenger side.

Screenshot_20250917_171549_Samsung Internet.jpg


Any advice on brand or aluminum vs. copper?
 
I know this day is coming for me, I feel your pain. Do you guys recommend replacing the AC piece while in there, it’s also 30 years old?
I guess if you want to? I don't know if I've ever had an evaporator fail on me.

I personally prefer the copper, but to each his own. I'm not sure on the heater hose outlet/inlet directions. It has been too long since I've looked at one. This is an OEM Ford part though.

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I would go with aluminum. As far as those brands, I have used UAC, Spectra, and Four Seasons before and never had issues with them. UAC and Four Seasons make all kinds of AC stuff and Spectra makes radiators and condensers.

A quick check on Rockauto shows the ones with both hoses angled the same way as being for a 4.6 car. I honestly can’t remember which way they are in my 97 and it is pouring rain right now, but double check which way yours are, and order the one that matches.
 
A quick check on Rockauto shows the ones with both hoses angled the same way as being for a 4.6 car. I honestly can’t remember which way they are in my 97 and it is pouring rain right now, but double check which way yours are, and order the one that matches.

Weird. So my car has a V8 core?! I wonder if that's not even original then...

20250917_174108.jpg
 
Hahaha! I just came in from outside to check mine for you and yes, they point in the same direction!

Joe, you know what this means? We both actually had V8s all along!

Thank you for checking!

I suspect Rockauto has them listed backwards.

And everybody else. Autozone, Advance,...; they all have them listed the same as Rockauto.

Probably the mix-up originated with Ford, and then everyone copied it.
 
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Yeah, my 91 was originally a 5.0, and I know those point in opposite directions. I’m also 90% sure the 90XR7 also has them pointing opposite directions. My bet would be they had to change it for the 4.6, and then probably decided in 96 to revamp the heater hoses to be able to use the same part for all of them.
 
I’m not a fan of those restrictors. First off, the whole cooling system is pressurized, so I can’t imagine it drops the pressure in the heater core significantly. Secondly, every car I have ever been in that had one of those restrictors you could hear the coolant gurgling through the heater core.
 
Don’t use a restrictor, if you want to do anything preventively for the next one’s longevity hose clamp a wire to one of the outlets tying it to the block to prevent electrolysis.


Pulling the column is worth it IMO. It’s 4 nuts, 1 bolt(u joint) and a few connectors to unplug. I don’t do it just to make the dash lighter to remove but minimize risk of damaging things(eg gouging the steering wheel leather on something sharp). Besides I don’t know about you but it takes me longer to pull the center console than it and that’s a piece of cake


As for the splayed out heater core outlets vs parallel, 4.6 cars are supposed to be parallel. Either one can be made to work though

I also agree, plan for two days. It is easier than it looks though and you might be surprised how fast it actually goes, at least I was the first time I did it. Getting the heater out of the box and back in is my least favorite part personally.


As for the AC evaporator, I don’t recommend touching it. You literally need to hack open the heater box and basically tape it back together to change it, there is no access port. So long as your AC is functioning there shouldn’t be much chance of it failing anyway
 
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My Crown Vic has the restrictor, but it is the fleet setup. I thought it was more to restrict flow through the core at sustained high RPMs. I don't think they put restrictors on the MN12s and I've never put one on in any of the heater cores I've replaced. I blew the heater core on the LTD wagon and I may put one on that when I get to it. Our '97 4.6 has both outlets pointing toward the passenger side.
 
Something that has always puzzled me is how over the years, so many people have never touched their heater cores and done little to maintain the cooling system yet had no issues. Then others do so much preventive maintenance, regular coolant changes, grounding the core etc. and have heater cores springing leaks all too often. And of course, everything in between. :zdunno:

Carry on...
 
Something that has always puzzled me is how over the years, so many people have never touched their heater cores and done little to maintain the cooling system yet had no issues. Then others do so much preventive maintenance, regular coolant changes, grounding the core etc. and have heater cores springing leaks all too often. And of course, everything in between. :zdunno:

Carry on...

Mines in between, I’ve admittedly been do as I say not as I do with coolant change intervals(I think I’ve fully changed the coolant 3 times in 20 years?), and I added the ground to the stock heater core that still is in it to date….(even though I’ve had the dash out 4 times) Oh and a few of my hoses are original(or at least partially as I used sections for Frankensteining)
 

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