Was driving on the interstate today and had the AC on. Got a little fixated on the ultra gage and was trying to see what I could do to change the engine temp, so I rolled down the window and shut the AC off. Didn't think much of it, got off the interstate and rolled up the window. Then I noticed a fine steam coming out of the dash vents...which eventually became a thicker, much more noticeable steam and fogged up the passenger side of the windshield. It didn't smell like coolant, or really anything. I pulled over and checked under the hood. Everything seemed normal; nothing on fire, coolant visible in the overflow. AC was draining pretty good under the car. Only thing I have really messed with lately was replaced the coolant overflow tank cap. Any thoughts?

 

I've had that happen and it was coolant. The coolant was steaming in the engine and being sucked into the vents. Fortunately I just had a loose hose clamp. But when I opened the engine I did see coolant. Maybe not your situation.

 

I get this als o in my other cars. It's normal.
From google...

Fog or Smoke coming out the air vents is caused by cold dry air coming in contact with warmer more moist air near the air conditioner. The air temperature near the unit is below Dew Point, this causes water vapor to form in the air and condense into water droplets causing the fog or smoke.

 

I've had this happen twice before. Both times it was a harbinger of a heater core about to let go. I always carry a splice fitting (in the old days, one of those plastic flush & fill fittings with the garden hose threads) and a couple of extra hose clamps for this reason.

 

Looking at heater cores. CARID has one for $40. They are $140 ish at AB. Is there any noticeable difference between the two other than $100?

 

I doubt there's any difference. All heater cores have been essentially the same for the last 50 years.

If you do end up swapping it, don't take the shortcut and cut the lines/replace with hoses. Doing the job right will ensure you don't have to do it again. It's not a lot of fun, but can be done in half a day.

In any case, I'd recommend you have a conclusive diagnosis before parts-swapping. (identify coolant leaking onto the floor or out the A/C drains, along with coolant loss from the overflow tank) Hate to see you make the swap only to find that the original was not the problem.

 

There's a difference, the CARid version will fail in short order, or will fit improperly. The genuine one won't.

Couple that with the enormous amount of labor involved in replacing a heater core, and suddenly the extra $100 you'll spend seems like a worthwhile endeavor.