I have a 99 Camry 4-cyl that does not maintain what I would call normal operating temps while driving. If the car is just idling in the driveway the temp gauge will come to midway (what I consider normal) and stay there. If I take the car out on the freeway that gauge will drop to cooler than normal. If I go back to city streets and am in stop and go conditions, the gauge will start to climb back towards the midpoint. This morning it was rather cold outside and so I was running the fan on hot to warm the inside. When I got to the freeway, I didn't have any hot air and the temp gauge never got above the Cold line. If I turned off the fan and got onto city streets it would climb a little over the cold line but as soon as I turned the fan on again it dropped down again.

Can I think that this is indicative of a thermostat stuck open or should I be looking elsewhere?

Thanks!

 

Midway or slightly below is normal.

Yes, start by checking the thermostat. It appears not to be adjusting to the coolant temp and allowing the coolant to become to cold by staying too open.

 

Thermostat, takes 5 min to replace.