Weird fogging

This morning before the sun came up, I got into my '89 Caprice 9C1 to head to work. It was clear and chilly with no (or very low) humidity. As soon as I got up to highway speed, all the windows fogged up (on the inside) and the defroster would not clear the glass, even though it was working fine. The fan had no problems and the heat was hot as ever. I've never had this problem, and this car has overall been a trouble-free gem since I became the second owner after a suburban fire company. It is never driven in rain or snow. This couldn't be a heater core problem if the heater works fine, could it? Thanks for the help.

It certainly can be a heater core issue ... when mine started to leak, it would fog up the right side of the windshield bad (plus you could smell the coolant in the car, that's a giveaway).

Also, these cars use the air conditioning when you select defrost ... the warmer air can fog up cold glass so they run cold air on it to remove the fog. If your A/C doesn't work, this could be why.

This morning before the sun came up, I got into my '89 Caprice 9C1 to head to work. It was clear and chilly with no (or very low) humidity. As soon as I got up to highway speed, all the windows fogged up (on the inside) and the defroster would not clear the glass, even though it was working fine. The fan had no problems and the heat was hot as ever. I've never had this problem, and this car has overall been a trouble-free gem since I became the second owner after a suburban fire company. It is never driven in rain or snow. This couldn't be a heater core problem if the heater works fine, could it? Thanks for the help.

it is also possible the drain tube is plugged up and trapped water in in the air box...this has happened to me ...all that tree pollen etc,,,pour water into the air intake grill see if it drains out the fire wall mounted air box drain tube ,,,use a piece of wire to clear out the drain tube,,,

Sounds like the windshield was still cold enough to draw the moisture out of the air being supplied by the defogger. Did the windshield continue to keep fogging up? Or did it clear up after the windshield/cabin heated up? I have had this happen to me before IMO the defogger draws in cold moisture, heats it somewhat then throws it onto a COLD windshield, warmed moisture vapor condenses back to H2O then forms the fog on the windshield. After the windshield warms up the fog clears up.

MY OPINION HOPE IT HELPS

I found today that the fog only goes away if I keep the defroster on and in the "Cold" position. Again, it's very strange that this problem would suddenly happen when the ventilation system still operates in all positions. As far as the drain tube being clogged, I seriously doubt that this is possible since the car is kept in the garage unless the weather is dry.

I found today that the fog only goes away if I keep the defroster on and in the "Cold" position. Again, it's very strange that this problem would suddenly happen when the ventilation system still operates in all positions. As far as the drain tube being clogged, I seriously doubt that this is possible since the car is kept in the garage unless the weather is dry.

with that info I would pressure test the heater core..using an air guage sealing the heater hoses apply 10 psi to the heatercore and see if it holds pressure..if not,,, end of story..

if only takes a very small amount of coolant to do this to your windshield...it will get worse...you will notice a film will form on the inside,,like cig smoking....