I live in Idaho where climate is currently cold and foggy or rainy. I had service back in September and noticed when using defrost or any kind of heat, my windows would fog up and make it very difficult to drive. Did this for a week, maybe two, then cleared up and was fine for the last few months. Recently had it serviced again and it is doing it again and it's even worse now than back in September. I've tried using various options outside circulation versus inside circulation, doesn't matter. Funny thing is I can start it up in the morning to warm up before taking kids to school and let it run with no people in the car, we pile in and windows start fogging up and no matter what temperature or setting I put it on, it does not defog the windows!

The smell of antifreeze is there as well but again only when the windows are fogged up.

Husband assumes it's the heater core and I tend to agree, but why would it function like this?

Vehicle is a 2003 Ford Expedition Eddie Bauer 2-wheel drive. Otherwise very good condition.

 

Welcome to the site.

Id be suspect of the heater core as well.
If you look on the passenger side foot/toe board area, pull the carpet back at the top near the heater box and look there for any leaking indication. It may just show as a moisture collection. As well as the underside of the truck, there will be a drain there that should have a nipple mounted on the box. Generally when a core leaks, it will present itself in those places. Sometime pressurizing the system will help pin point the cause. Most often steam on the windshield is the core.

 

It doesn't seem to fog up though when it's idling still, only when we try to run it / drive it down the road with people in the car.

I also checked for leaks and see no signs of a leak.

 

As the engine heats, pressure inside the cooling stream increases. Higher RPM's tend to increase flow volume and pressures. Could be the reason it's seems more intermittent. Best way to check for leaks is to have the system pressure tested. More so before any work is performed. But if it's steaming the windshield, the only part on the trucks interior that contains coolant is the core. If you smell it, visually see the moisture on the windshield, I'd certainly be suspect of the core.