I have a 1998 four cylinder Toyota Camry. The heater wasn't working so I took it to a Midas shop. They said all their temperature measurements were normal and didn't know what was wrong. I decided to replace the thermostat my self to see if that would help and it totally fixed it! It worked for about two weeks and now it doesn't work again. I flushed the radiator thinking maybe the cooling system was clogged but everything came out clean and it didn't fix the problem. After the drain and flush, I checked the manual to see my coolant capacity. It was 6.9 L. I could barely get 3 L in the radiator before it was full and wouldn't accept anymore. Any thoughts.

 

Automatic or manual climate system?

Did you flush the heater core, check the heater control coolant valve, check that the air mix door is working?

 

I'll check those and report back