I have a 2000 Celica GTS that is blowing cold air from the heater consistently. The engine temp is totally normal just no heat. Summer is over. Now it’s time to figure this out. Any advice/suggestions?
Locate the supply valve in the coolant lines and make sure it switches from off to on when you turn the temp switch to hot. It has a cable in the back of the console from the switch to the valve; make sure it is connected.
Sometimes radio installations disconnect those cables.
Could be that when you idle the water pump doesn't have enough flow to move hot air around the motor, but at RPM it can move. But normally a bad water pump is indicated by overheating at idle, not cooling. Could also be a failed thermostat, but generally a stuck open thermo will cause the engine to never reach operating temperatures.
Do you have normal operating temperatures on your idiot gauge (dash gauge) or does it also go down as you idle?
If you had a leaking hose you would have an overheating issue with the engine as you would be losing coolant.
I think its a circulation issue - if is is hot but then cools down, as the fan blows across the heater core it is cooling down the coolant in the core, but if it starts hot or gets hot again makes me think there is limited coolant circulation.