Corvettetodd

I noticed as it gets colder here in Southern California (40 degrees at night) that when I turn the heater on the windows almost immediatly fog up. This only happen when it is cold outside.
I know the first thing to suspect is the heater core leaking, but it only fogs up when it is cold outside,there is no coolant smell at all, and I dont seem to be losing coolant. I know the smell of a bad heater core from a C4 that I had with a bad heater core.. it also leaves a film on the windows. In short, I do not think the heater core is leaking.

Is there anything else that could be causing this? Maybe bad air circulation?

Jims66

I know you said you don't think the heater core is leaking... but, is your carpet wet under the mat on the passengers side ???

LB66383

There has to be moisture in the interior somewhere. Have the t-tops or some other area leaked during a rainstorm or car wash?

Corvettetodd

Nope... bone dry under the passenger side mat...

OMF

I see you have a '71......I'm not sure at all about them, but does your car have an air recirculation feature? If it does, move the selector to allow fresh air to blend with the heated air and your troubles will go away. All that breathing your doing is creating to much humidity for the air, and its condensing on the windows.

Kevin68

Is it dewy in the morning? Do you park in the garage? What is the temp and humidity when you park the night before? Do you live in a valley (where cool moist air will tend to settle on still mornings)?

If you park in the garage with in warm moist air sealed in the car, then pull out in cool air (which does not hold as much moisture), the blast of cool air can cause the moisture in the air in the car to condense. To check this, park with the windows down (if garaged) and ride for the first 30 seconds or so with the windows down to get the air in the car to ambient temp and humidity.

Or, as others have said, you have a water leak. Either way, the atmophere in the car is supersaturated with moisture at the cool morning temps.

sjr1971

pin hole leak in the heater core

7T1vette

There is some 'standing' water in the heater plenum. It could be the heater core or it could be left over water from a rain or a car wash which leaked in via wherever. It only does that when it's "cold", or whenever the dew point is high enough for fogging to occur. If no coolant smell, it's likely due to a small water leak at the windshield or cowl area.

Corvettetodd

They do not fog when I drive the car after being in the garage.. It only happens when the car is outside in the cold weather and I start it and drive it away. They do fog a little before turning the heater/defroster on, but really fog when the heat/defrost is turned on. The defrosters do clear the windshield after about 15 minutes and the windshield gets warm.

I am assuming the heater core is original, but there are no other signs of heater core leak. It has been a year and about 5,000 miles since I put the engine back in and refilled the radiator.. It is not low on coolant at all.
I am sure that even a pinhole leak would leak most of the coolant out within 5,000 miles of driving..

Z-man

IMO, Kevin68 has the right idea. In any other part of the world where they have Winter, drivers know not to direct air flow to the windows for the first few minutes of driving. This is because there can be a bit of moisture (dew) on the heater core until the coolant circulation makes it evaporate. It happens with all cars - not just your Vette.

If you ever travel outside your State when temps are below freezing outside, you'd find out that, not only will the window fog up, but it will form a sheet of solid ice on the inside of the window. The ice is hard to melt or scrape off, especially at, say -15 or so. It only takes one or two times for this to happen before folks remember to leave the defroster off for a few minutes...

TEXASDESERTVETTE

Another trick that you may find helpful is to turn on the A/C unit and adjust the Heat as necessary to the desired level...this will dry the air... the newer motorcar automatic systems do this in the defrost mode setting.

73, Dark Blue 454

That's a classic symptom of a heater core leak. A small leak can really fog with windshield with water vapor. The coolant could be leaking away from the carpet somewhere.

69elky

I had same problem on my 79 and it was my heater core,pin hole leaks and when system turned on caused fog condition on windows with no coolant smell was noticed.

Corvettetodd

If the heater core was leaking wouldnt it fog the windows all of the time, not just when it is colder outside?

It was about 70 degrees today and I turned on the heater and defrost for about 15 minutes to see if I could get them to fog up, and they didn't fog up at all..

I am guessing that it is the moisture in the air at night that is doing it..

noonie

Trick I learned years ago is to rinse the windshield with a hose first thing and it won't fog up or condense in southern high dewpoint climates when it gets cooler out.

Kevin68

...although, I don't think a leaking heater core must always fog the windows, so that is not ruled out. It's all about relative humidity in the ducts vs. ambient relative humidity. Dry ambient air may be able to keep the moisture from the ducts in solution, whereas wet ambient air may not. But as you said, you are not losing coolant after many months, so I wouldn't start pulling your heater core yet.

Another thought, maybe condensation is collecting in the duct work and you have no leaks (heater core, windshield, or anywhere else). That uncirculated moist air in the ducts is warmer and can hold more moisture, then you circulate cool air on an cool windshield, and voila, a new place to for the moist air to condense....foggy windows.

OTOH, how do we get the air in the ducts to condense without becoming cooler than the outside air? hmmmm. AC? Are your windows dewy in the am? Keep you eye on the local weather, temp and relative humidity, keeping in mind that protected valleys can be significantly different. It's an experiment that's free and may shed some light on the problem. Do you have a nerdy neighbor that has an outside thermometer and and humidity gauge?

....or it's global warming

AzMotorhead

Simply put, the glass is cooler than the ambient temp when you direct air towards the glass and moisture in the air condenses on the glass.

Just like how your glass of chilled beverage get condensation on it when there is humidity in the air.
You're in So Cal by the coast= there's moisture in the air.

It only does it on startup becase the Heater doesn't have any heat in it yet to warm the windshield. A/c hasn't had time to dry the air.
This is a normal condition.
Wait till you're driving in a rainstorm on a cool day you'll really get some fog then. Thats why heater delete on a driver is not a good idea

vette427-sbc

yes, i definately think it is the moisture in the air at night....i would try the heater/defrost trick when it gets humid out and see what happens to guarantee its moisture...deff a minor issue

Corvettetodd

Thanks for all or your input.... I think the moisture in the air is doing it... the car is kept in the garage at all times, so it is not noticed in the morning because the car is warmer ..


I will continue to monitor the coolant level.. I know the heater core replacement is a PIA on this AC car, but it cant be any worse than what I have done to the car already (trailing arms come to mind) .
I may replace it one day just because I know it is 40 years old and coolant on my new carpets would not be good..

I understand the Genuine GM units are still available.. I may order one and put it away..