Hey all,
I just got done with a engine swap on my '86, now the windows fog up when the heat is on and have to run the defrost w/ full fan just to keep the indsheild clear (witch dose not work with out cleaning the windsheild several times and it will start fogging almost imedatly) or turn the heat down to half way or less with defrost on (will still do it but will clear up on its own and is not as bad as full heat. This takes about 20 miles or so to be completely cleared up),I can use anyhing else but defrost all the time.
Before the engine swap I did not have this problem the only thing that was done to the interior air system is, 1. the fan was removed and put back in. 2. all the componets were removed from the engine compartment. 3. the engine carpartment was painted. 4. all the componets were cleaned out. 5. the space infrount of the windsheild was cleaned out. The interior componets were not touched (not ready to pull that apart yet). the car has lots of heat, no coolant leaks, no dampness in the interior, no puddles under the car, the car has about 600 miles on the new engine. The car still has A/C but I have not put freon in it yet so its not working.

 

Is the fog slightly oily to the touch? Does it seem more of a residue than a fog?

If so you may need a heater core.

 

Not oily no residue it like when you breath on a cold window

 

T-top car? Sounds like you have moisture in the interior. The heat will evaporate it up and it's going to condense on the coldest thing in your car, the window.

 

Sounds like a bad heater core to me...does the heat smell like maple syrup? Any drips in the passenger area front carpet?

 

hard top car, no smell, no water on the floor board, no water loss in the coolant system, no water dripping under the car. she is keep in the shop kinda temp controled. it did not start doing it until the weather started cooling off. I'm using a white towel to clean it as I go so I can see and the towel is not being discolored.
I always thought a heater core would leave a noticable amount of water on the floor board but I have none.

 

With all the above, It has to be condensation. Air temps and concrete floor sweat could cause this, If you park inside a shop leave the windows down about a inch and see if it continues. I have the same issue with my Jeep occasionally......