Heater Core Woes

Title says it best. Yesterday morning I turned on the heater, you know, like anyone would do on a chily 25* morning. And I get a nice smell of coolant and some condensation on the windows, and from cycling the different hvac settings including using the circulating air setting. Im pretty sure ive narrowed it to the heater core. No coolant leak in the engine bay, I did a very thorough sweep of all connections and the radiator.

Now, in a perfect scenario, I would like to just outright replace it. But being the busiest time of the year with school, thanksgiving, etc, and it only getting colder from here im thinking of adding in a sealer. Or just bypass the heater core and pickup a 12v electric heater and heated blanket:giggle:.

Ive never considered a sealer until now, but also don't want to create any unnecessary repairs for myself. Any opinions/recommendations are welcome.

We were getting a similar smell it ended up being a bad radiator cap which was showing no signs of leaking but once replaced off of a suggestion from a friend solved the problem. Definitely go Ac delco. Cant guarantee that will fix it but it is the problem we ended up having with same symptoms. Ive never heard of a heater core going in a H3 yet unlike other platforms we work on that it is more common.

The stop-leak is probably the last course of action I would advise. You're just adding more 'gunk' to the cooling system. Way back when ...when I was a young car owner/driver, I tried it a few times on heater cores and it really didn't work. The reason probably being that it only takes a very small amount of leakage to be obnoxious. Sometimes it seems serious and there's barely any traces of leaks on a heater core. The nose is very sensitive. From my experience, a heater core with a small leak can stay in that state for years literally.

If it were my vehicle I would probably live with it until you can get past the holidays and get it replaced. If the odor is obnoxious, you could bypass it. Those little electric heaters that plug into the ciggerette lighter are practically worthless. There are guys on the Jeep forum who've tried them and post all the time. A good friend of mine had a heater issue on a long trip, tried one, says the same thing.

What you could do temporarily, is pre-heat your car in the morning using a home space heater on an extension cord. Then you may not need the heat on your commute. Course it only gets you there. But...

No heater core, no effective defrost.
Other than condensation on window and coolant smell through the vents, another sign is coolant leaking into the pass compartment. But if man made it will eventually fail.

Not a fun thing to replace; therefore, you should do some diagnosis prior to pulling out the I/P.

We have a development. Noticed the smell/ condensation a couple days ago, this morning I noticed a small puddle of coolant on the ground. Does the heater core sit in a box so that it can drain out in this scenario?


I’m going to recheck the 13 y/o heater core fittings.

Jeepwalker said:

The nose is very sensitive. From my experience, a heater core with a small leak can stay in that state for years literally.

If it were my vehicle I would probably live with it until you can get past the holidays and get it replaced. If the odor is obnoxious, you could bypass it. Those little electric heaters that plug into the ciggerette lighter are practically worthless. There are guys on the Jeep forum who've tried them and post all the time. A good friend of mine had a heater issue on a long trip, tried one, says the same thing.

What you could do temporarily, is pre-heat your car in the morning using a home space heater on an extension cord. Then you may not need the heat on your commute. Course it only gets you there. But...

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That is most likely what ill be doing, living with it until post holiday. The nose is very sensitive indeed, however its to the point where the odor is very strong and makes my eyes water. Reading into what the CDC says for breathing in Ethaline Glycol, they say it can inflammation in the lungs (Can go way more specific, but don't feel like its needed) when heated and circulated. Id prefer to not have any health risks over being a little cold this winter.

atvspeed4 said:

We were getting a similar smell it ended up being a bad radiator cap which was showing no signs of leaking but once replaced off of a suggestion from a friend solved the problem. Definitely go Ac delco. Cant guarantee that will fix it but it is the problem we ended up having with same symptoms. Ive never heard of a heater core going in a H3 yet unlike other platforms we work on that it is more common.

Click to expand...

I had this issue this past summer, noticed a nice ring of dried up coolant around the cap, replaced it with an ac delco, and fixed my mysterious coolant smell. My smell in the cabin is strong and makes it tough to keep the heat on. So much that I prefer turning it off and wearing a heavier jacket.

f5moab said:

No heater core, no effective defrost.
Other than condensation on window and coolant smell through the vents, another sign is coolant leaking into the pass compartment. But if man made it will eventually fail.

Not a fun thing to replace; therefore, you should do some diagnosis prior to pulling out the I/P.

Click to expand...

Havent had any coolant leak into the passenger compartment just yet. Ill end up replacing the thing myself, while I have the dash apart itll be a good time to run wiring for accessories and organizing the wiring I already have running through the dash.

That's if it is the core, want it to be undeniable before I dive into that project.

One thing you might want to look at closely is the top header on the radiator, they are noted to crack and allow leakage (small amount) and due to the H3's front skid plate, it can drop on to the plate and never be a puddle where you park. What coolant is there will just fly off when driven.
When my radiator started leaking I noticed the smell, never notice a drop on the ground, but I pulled off the front skid plate and there was coolant.

Everything you've described is 'classic' heater core failure symptoms. I've had a few fail and I understand how miserable it can be. I wouldn't want to inhale the fumes either. I used to have a car where I had to drive it to work with the windows down for a couple months (in a midwest winter!), before I could get at it.

A couple things you might do to narrow it down a little more (if you haven't already) is:
1) Inspect the hoses and inlet, along the firewall with a flashlight, to eliminate the possiblility of a leaking hose clamp at the firewall.
2) Remove the plastic scuff sill plates on each side (they just 'pop' up by hand or screwdriver), and put your hand under the carpet in as close to the tunnel as you can and feel for wetness. If it's leaking most likely the carpet could be wet with antifreeze.

I did a heater core replacement last winter (non H3), that involved removing the entire dash, which is never fun. They're usually not as bad as ya think they'll be going into them, it's just remove piece by piece. There's probably a video out there how to do it. If you're THAT busy, maybe shop around for pricing ...you never know. I find the H3 to be extremely logical and fairly simple to work on. My heater core day is probably not that far away either, ha ha!

I could pull up the procedure on my other old windows 7 laptop; however, from memory it does entail removing the entire dash assembly; not the cluster and that is one major job. If it was a dash cluster that on the H3 is about five minutes.

But I still say diagnose and if NO coolant inside the vehicle I would really inspect that radiator very well (especially around the header where a crack is a known problem...mine went at 51K). And those skid panels do a great (hell, fantastic) job of hiding small coolant leaks. My first indication when I started to smell coolant was it was a heater core; further inspection proved it wasn't.

f5moab said:

One thing you might want to look at closely is the top header on the radiator, they are noted to crack and allow leakage (small amount) and due to the H3's front skid plate, it can drop on to the plate and never be a puddle where you park. What coolant is there will just fly off when driven.
When my radiator started leaking I noticed the smell, never notice a drop on the ground, but I pulled off the front skid plate and there was coolant.

Click to expand...

I did a very thorough examination of the radiator, even went as far as removing the shroud to get a complete look. Bone dry, no seepage in the usual and unusual places. That happened to the previous radiator, cracked up top right at the support.

Jeepwalker said:

Everything you've described is 'classic' heater core failure symptoms. I've had a few fail and I understand how miserable it can be. I wouldn't want to inhale the fumes either. I used to have a car where I had to drive it to work with the windows down for a couple months (in a midwest winter!), before I could get at it.

A couple things you might do to narrow it down a little more (if you haven't already) is:
1) Inspect the hoses and inlet, along the firewall with a flashlight, to eliminate the possiblility of a leaking hose clamp at the firewall.
2) Remove the plastic scuff sill plates on each side (they just 'pop' up by hand or screwdriver), and put your hand under the carpet in as close to the tunnel as you can and feel for wetness. If it's leaking most likely the carpet could be wet with antifreeze.

I did a heater core replacement last winter (non H3), that involved removing the entire dash, which is never fun. They're usually not as bad as ya think they'll be going into them, it's just remove piece by piece. There's probably a video out there how to do it. If you're THAT busy, maybe shop around for pricing ...you never know. I find the H3 to be extremely logical and fairly simple to work on. My heater core day is probably not that far away either, ha ha!

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Called 3 good local shops that I would trust. All 3 quoted 10+ hours of labor and around 900$ to do the job. Ive asked a couple people on the facebook pages that had recently undertaken the project, they said the A/C also had to be evacuated due to having to remove the ac evap core. I guess Ill replace that thing as well while im in there.

Also a GREAT time to drill out the sunroof grommets. Have to look at the plus side.:giggle:

f5moab said:

I could pull up the procedure on my other old windows 7 laptop; however, from memory it does entail removing the entire dash assembly; not the cluster and that is one major job. If it was a dash cluster that on the H3 is about five minutes.

But I still say diagnose and if NO coolant inside the vehicle I would really inspect that radiator very well (especially around the header where a crack is a known problem...mine went at 51K). And those skid panels do a great (hell, fantastic) job of hiding small coolant leaks. My first indication when I started to smell coolant was it was a heater core; further inspection proved it wasn't.

Click to expand...

If you still have this procedure it would be very, very helpful. I was going to grab some scotch tape and a sharpie and mark each part in the order it was taken off. Also, I believe GM engineering did a great job designing the HVAC in the H3, they locked away the heater core in a box and added a drain. No coolant in the interior for me! Ill post some pictures I took of the drain later on today.

While $900 is a lot of money, but it's also a lot of work, esp if it includes re-evacuating and charging the AC (and the price of a new core?). Depending on your funds and situation it might be well-spent if they're a 'good' shop. I've done a few remove-dash heater core jobs and 10 hours sound about right, actually quicker than I'm sure I would take. Plus ya have to figure breaking the heater box apart to replace the core and seal it back up. I would have them (if you go that route) drain and install new coolant as well and adjust to the pH required by GM. Coolant pH changes over time and a high (or Low) pH can 'eat' away the heater core because it's very thin metal. Aluminum has a very narrow pH range (tolerance) compared to copper cores. I'm convinced a lot of guys who complain their new heater core flaked-out in 2 years ran a high pH and it ate itself out. Coolant pH should be checked with a digital meter (test strips aren't very accurate). Read up on it and you'll see what I mean. I worked in industry and it's well-known, especially on equipment needs to run 24/7 where downtime can cost into the 6-figures every half hour. The good thing about the H3 is I think the entire cooling system is aluminum, so not many mixed metals (which have different 'happy' pH levels). Don't overlook the little things.

I'm not sure why the need to replace the evap core? I'm not sure it's a common fail item. The AC is a 'closed' system, not like a bunch of crud flowing through it like could be the case with a heater core. But if you do, go Delco. And if there's an inline fine-mesh filter screen (I don't know if the H3 has one) ..probably should clean or replace it. Same with the heater core (go Delco).

Good luck, Let us know how it goes.

Note: This digital pH test pen includes the 3x solution packs to calibrate with (meaning it's more accurate than the 2-pak kits):
https://www.ebay.com/itm/Digital-PH-Meter-Portable-Pool-Water-Aquarium-Urine-Pen-Tester-Auto-Calibration/282076114721

Currently have this in both H3's.
Simple and cheap. Dark garage and a black light . I was happy it was just the quick connector at the firewall
https://smile.amazon.com/ACDelco-10...ds=ac+delco+coolant+dye&qid=1574455663&sr=8-6

Dark garage and black light....sounds a bit kinky; speedy!:giggle:

I replaced mine a few years ago. Had the same symptoms as what you mentioned. It was not a fun repair.


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With the whole thing out, is it possible to add an incab filter for the hvac? I looked at the diagrams and couldnt tell where the H3 pulls the air inside the cabin

You can see right below the EVAP core connection is where the drain is for the heater core. When it was bad, I could see coolant running out of there.

I have done it due to AC evap replacement.
Replaced heater core just because I was there.

Not a fun job, I wasn’t in a hurry so it took few days.
Whole dash needs to be removed and then complete HVAC housing. A/C needs to be recovered and recharged afterwards.


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I lost my heat twice last winter but It turned out to be the actuator underneath the dash on the drivers' side. I had to replace it twice but now heat is working again. I guess I got lucky. Sorry youre going through this. I hope it gets resolved for you my friend.

My PTSD is kicking in. Did heater core on ZR2 many years ago. I'm still flummoxed as to how it held together with a half a Dixie cup of screws left over (I'm told that's how you know it was done right). Also curious how the previous owner managed to drop a .223 round into the vent, which I later found in the baffle box when doing the R&R.

Please don't tell me that the heater core is a common R&R on these H3s, I've only had to do one in my lifetime and all of my vehicles have WELL over 150K by the time I get rid of them.

H3 heater core replacement isn’t very common but an S10 ZR2 heater core is a piece of cake compared to the H3.