Foggy Windows w/ Heat… Bad Heater Core?

[David Heine]

Vehicle: 2008 Chevy Cobalt LT
Milage: 104,000

While driving home last week, I had the heat/defroster turned on & within 1 – 2 minutes there was so much condensation on my windows and windshield that I could barely see anything. I stopped to clean them off using glass cleaner, and within 2 more minutes of continuing my commute home the windows were fogged up again, like a clear oil-like film on the glass. Cleaned ’em off again, left the heat off this time, and they were fine. After a few minutes of driving in 15 degree weather and not being able to feel my hands, I turned the heat back on (low setting). Boom, windows were fogged up again.

Looked up what it could be, and everything points to a bad heater core, except for all of the other common symptoms. My heat works fine (nice and hot), I’m not noticing a loss of coolant, I’m not noticing any wet spots on the passenger side floor or under the car, I don’t really smell any fluids, etc.

So is the heater core still the culprit? Seeing how big/difficult of a project it is to replace it, I was wondering if anyone can provide any other possible causes or fixes before begin disassembling my entire dash.

Thanks in advance!

Topic

[Billy]

Yep, heater core. Sometimes there’s an easy way in if you research.

[David Heine]

Thanks for the fast response! I watched some videos regarding how to disassemble the dash to get to the heater core in a Cobalt. It looks like it’s buried deep inside the dash, people needing to take out the entire dash, some even needing to drop the steering column. I’m not looking forward to this! Lol

Since my heat is still hot and I’m not noticing any leaks, do you think it could possibly be anything else? Thanks again for the quick response!

[Mike]

With your description of an oil-like film, sorry to say it does sound like a heater core leak.
The only other possibilities that come to mind are water leaks into the interior, especially if it has a sunroof.
The drainage tubes can become disconnected, so the water soaks the underside of the carpeting.
Often it does not feel wet to the touch on the carpeting side.
A clogged evaporator box drainage tube would be noticed as a wet spot on the carpeting.
Same for windshield leaks.

[David Heine]

As unfortunate as it is that it really might be the heater core after all, I appreciate your response! While they aren’t very expensive for the heater core itself, do you think the whole thing should just be replaced?

I know this is probably a dumb question (so please go easy on me! Lol) but would something like BlueDevil Pour-N-Go work to chemically seal a leak in the heater core (assuming its a small leak)?

Thanks again for your help!

[Richard Kirshy]

Just a thought..
Might have a look at the drain to see if it is clogged.

[Mike]

Pour and go is designed for leaking head gaskets.
I would not try it on a heater core leak.
The problem with using any stop leak product is twofold.
You may stop the initial leak but another may follow shortly thereafter.
More importantly, you could clog up the heater core and then have absolutely no heat.

[Dave Tidman]

Just a thought, but is your HVAC on recirculate? Or maybe the recirculate door is stuck? It’s strange that you are not losing coolant.if you have a heater core leak. What happens if you just use defrost?

[zero]

Before you assume the worst, stick to the basics and diagnose it before you make plans, buy parts or rip the car apart.

Your windshield needs to be properly cleaned. Once the fog up, dirt sticks to the glass attracting more fog and on and on. Any open bottles of water or other drinks need to be removed. If the floor mats are wet or damp, pull them out and dry them off. A small amount of water in there can easily fog up the windshield in lower temperatures.

[David Heine]

@Bonnieman I agree completely, sooner or later it is going to have to be replaced even if a chemical sealant fixes it for the time being. I was just curious if products like BlueDevil Radiator or Block Seal are risky to use as possible temporary fixes for a leak in the heater core, or if they are known to do more damage than good. I definitely plan to replace it regardless once the weather outside here in Western NY warms up a little (ironic planning, I know :huh: )

@dtidman I thought of that too and did some testing last night on my drive home. It happens on on recirculate, it happens on just defrost, it happens when the fans aren’t on and the vent is just open, etc. I’ve tried everything, it’s frustrating! Lol

[Mike]

Not sure if you read the line below that where I said, “More importantly, you could clog up the heater core and then have absolutely no heat. ”
I said it because heater cores are notorious for clogging on their own.
With temps here below freezing, I feel your pain.
Nothing like car problems that start when temps are wicked.

[David Heine]

I did see that, I hope it didn’t come off as ignoring it as you’ve been a huge help. At this point it sounds like my only two options are finding a warm garage to use for a day or two next weekend to replace it, or risk using a product like Block Seal and potentially have to replace it immediately anyways lol

Thanks again for all the help!

[Mike]

No worries–it’s all good.
Hope your weather warms up for the next weekend.
In the meantime, if it isn’t raining/snowing, cracking open a window may help.

[Wallace Brake]

I think dtidman might have a good suggestion – check your A/C system. It is designed to take the moisture out of your air and comes on automatically on most new cars as soon as you go to defrost. If your A/C is on the fritz then you could get the fogged windshield with no leaks from your heater core.

[Author]

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