I'm trying to isolate a problem I'm having with my heater. I have a 97 ES300 and the heater seams to only work when the car is moving. If I warm it up in park it will only blow cold air until I start driving it. Also if I come to a stop light it will begin to blow cold air as well. I thought it might be the thermostat but I'm not sure as I've never seen these symptoms before. Any help would be greatly appreciated trying not to spend money if I don't have too.

 

First, try just shutting off the A/C via the switch in the climate control. If the LED is off, the A/C is off. All the automatic climate control stuff will continue to work.
My guess is that you had a temperature sensor go out somewhere maybe. Or a damper door under the dash somewhere quit moving.
Good Luck!

 

You mention that you think there could be a issue with the thermostat. What is the temp gauge doing? It should move to it's normal position and stay there with very little change, driving or parked. If it is dropping back down to the cold end of the gauge then yes, it is time to replace the thermostat.

 

if the car sits too long while running it will increase in temperature and over heat

 

Sounds like a circulation problem. Any idea on the service history of this car? I would recomend a good rad flush, use one of the two part kits (one is a acid flush, the other neutralizes the first). Catch the stuff that comes out of the first flush, you want to see what comes out of the rad. Of course make sure that your heater is set to max heat while you're doing the flush.

Be aware that if you system is too corroded, this flush might remove enough corrosion to open up leaks. You could possibly end up with bigger headaches than what you have right now.