Please help...so when I turn on my heater a mist of fog begins to cover the windshield. The defroster will not eliminate the mist.  The air condition works great ( no mist/fog) and there is no evidence of water on the passenger side floor. This a 1976 Corvette . This is my first time on the forum and the stuff you guys know is VERY impressive. I hope you can/will help me 'cause my wife does like the heat on cool Spring and Fall morning drives. Anything???

Without a doubt a leaky heater core. I'll bet that your passenger side carpeting may be dampened with antifreeze also.

Thanks for the reply...Zero water on passenger side, that's why I can't explain the problem. Pulled up the carpet...no moisture.  Would there be evidence of leakage in another area??  Again thanks for you input.

Look well try and see if the cool ac air cleans it off...leave the heat off and turn on the ac only..meaning the evaporator is not getting used by the VIR...the blend door might not be working letting the evaporator do its job...there may be a vacuum leak somewhere so its not working to remove the water in the air...if you're certain the ac is working and fully charged it should remove the water in the air when in use..I know mine gets fogged fast if I have the top up and don't use the ac to clear it off and it does fog up as soon as I turn on any air on the windshield...so bad that I can't see out of the car..

Ok..so I read your post again and it says ac on no mist...so that does point to the blend door not working right...give that a look and see what you find.

With nothing on the floor, I think Rich is on the right track - assuming then no antifreeze odor at all in the passenger compartment.

first of all.... Are you losing anti freeze? Do a presure check on the heater core thru the supply and return hoses

|UPDATED|3/11/2012 5:51:41 PM (AZT)|/UPDATED|

Couple o comments...
With the A/C running, you shouldn't be getting any airflow to the top, anyway, so misting wouldn't be an issue. Also, with the Defrost ON, the A/C compressor should be running, which should eliminate the fogging...as it's supposed to do.
High humidity can cause this in certain parts of the country, and could be considered normal. There could also be some standing water in the cowl/vent area that can cause it, if the cowl drains are stopped up. However...using the Defrost should eliminate the fogging almost immediately. I kinda hafta agree wid Rich, in that the diverter doors may not be moving as they should be, whether due to a vacuum problem, a bent/warped door, or a linkage issue...

IMHO. I would still lean towards the heater core. Often when they start to leak its ever so slight in the middle of the core. Hence not enough to spill out on the carpet. Plus some of the coolant will fall into the evap drain tray and run out the drain hole on the firewall side. The problem your describing is exactly the way my 71 started out leaking until she finally let go. The mist started on the corner of the pass. window. If mist is forming all the time this is what I would expect. If only when its humid out I would agree with possible AC issue. What I would do is bypass the heater temporally and run the heater for a few days to dry out the core. Then run the air for a while see what it does. If the mist goes away with just the heater on I would lean towards the heater core.
Oh and to mention I have replaced hundreds of heater cores on various models that did the same thing. Not always bad enough leak to make a foot washer on the pass. side. But like mentioned, its kind of a pretty intense job to replace the core, so I would be 100% before tearing into it.

My .02, "T"

Heater cores can indeed seep enough to cause this, and not show up as leakage on the floor, for sure. Ya wanna be sure that's what it is before ripping the dash apart. I agree with bypassing the core to find out, and, as Terry points out, you'll need to run it a while before all of the coolant dries up off of the core, so it's not just a by-pass and check deal...if it is the core, it will still fog up for a while, until the coolant dries up...
This is when having a bore/video scope is very useful. You can snake a borescope up in there, and have a looky-loo, to see if the core is moist/wet.