Originally Posted by deadlast
No, it seems to stay at a constant level...just luke warm.
Also, coolant overflow was down to a bit over min recently, but topped that up and saw no improvement.
Bogey thermostat maybe??
If it was a faulty thermostat staying open, when you stop and let the car idle the internal heater would start to warm up.
There could also be an airlock in the internal heater, depending on the design of the system the air might need to be bleed off (just like your radiators at home). Some one else here might know if the corrola has a bleeder valve on the internal heater, it would typically be on the pipes in the engine bay before they pass through the firewall.
If there is air in the heater matrix you more than likely have a small leak somewhere in the system.