Heater & defroster works fine but inside car windows keep fogging

If the fog is oily, then your heater core is leaking and the blower is blowing coolant onto your windshield

Assuming that it is working in frigid conditions, sometimes The
Ac works with the defroster to remove moisture and the fog on the inside windshield. So, turn the defroster of unless you need it when the windshield fogs. You may be able to keep the window clear by opening the outside air vent and increasing the heater temperature.

If it was me, the first thing I'd do is go check my fluid level in my radiator. I'd add fresh anti-freeze, if I hadn't done so in a year or more. Your defrosters won't work properly if you are too low on fluids in your radiator, OR your thermostat is malfunctioning. Replacing your thermostat might be a good job for a mechanic. Good luck.

This may be due to a bad heater core or dirty core with a plugged water drain, first see if the a/c works. turn it on and see if the air gets cold and if the ac compressor works.

The way the defrost works to keep windows from fogging up is that when the front defroster is tuned on the ac should also be on, on most cars when you go to defrost it will turn ac on automatic. the system then uses hot and cold air to remove the moisture in the air to avoid window fogging. if your a/c is not working then that may cause window to fog, but if you are also getting a smell then the drain that lets out condensation may be plugged and then water builds up and also mold from always being wet and when heat is on you smell that.


i want you to run the defrost and when windows fog up wipe window with hand and see if it is greasy . if it feels oily then you may have a bad heater core, the core leaks coolant out and steam and causing windows to fog up with a flim of grease.

Defrost

If defrost doesn't remove the fog, or it gets worse with defrost on, you may have to have the heater core replaced.

Make sure the vent system is set to draw air from outside. If it is set to recirculate the same air that's inside the vehicle without drawing fresh air from outside, the resulting humidity that builds up inside the vehicle can fog the windows even when you run the defrost. You can also try opening the windows a bit to get some more air moving inside.
If it still fogs when the vents are set properly, you may have a leaky heater core. The heater core is a small radiator that is located under the dash that the heater system blows air through to heat the inside of your car and provide heat to defrost the windshield. The fog may be coming from the heater core leaking hot coolant steam that's being circulated through the defrost system. If this is so, you will need to replace the heater core.

If it smells like antifreeze, and leaves a film on windshield, it's probably a leaking heater core, but if not, your drain tube/passage for heater/AC duct-work, is plugged and not allowing moisture to drain out. Not 100% sure where drains are on that but, I would look real close at lower firewall in engine compartment,opposite duct-work, as well as under truck near front floor, for tubes sticking out, and carefully and gently, run a pipe cleaner or similar in & out to clean, until moisture starts to run out.

That is normally a classic symtom the Heater core has gone, Another is normally lost of coolant or smelling of antifreeze

This can be caused by a heater core that is leaking inside the car, one way to determine if that's the problem, is if you can smell antifreeze. It is a 'sweet' smell, and is quite noticeable. If u don't notice this, I have to say this is a common problem w/ Hondas. I've had a civic, 2 accords (currently have a 92), and 2 Preludes, they are great cars, but they all suffer from fogged up windows in cold weather. One reason is that they are quite air tight, and the older models don't have the greatest defroster system (weak). Try and keep a window cracked open slightly, see if that helps.

your heater core has probably sprung a leak.does the fog smell kinda sweet?....check the floor in the front where the floor mats are for moisture or even puddles of antifreeze.(the replacement of a heater core can be very expensive as the entire dash usually needs to be removed to access the heater core)also look under the engine near the firewall for antifreeze drips.