96 escort overheating during idle when air is on.

Does the (radiator) fan work?

Oh yeah, the fan is working, but does it have two speeds?

Check your Thermostat, sounds like it has failed, and is only partially opened, or none at all. Whatever it is, not enough coolant is passing through the radiator.

If the fluid levels/condition are fine, the fan works, etc (there are six? troubleshooting steps in your Hanyes manual for this problem), the next step is to read the codes at your local AutoZone or friendly mechanic.

My car was doing this and I was not able to work through the troubleshooting steps, but I took it to my mechanic and it turned out an electronic doohickey had failed, the one that tells the fan to kick in.

The reason it's 'fine' on the highway is because you're moving fast enough that you're cooling the engine that way. :)

The electronic doohickey cost me $48 plus $25 labor (I assume it was a pop in and out of a thing).