What causes Heater the

also, when you are in idle and just sitting around a parking lot, the coolant system has NO clue what your doing because it's job really is to be pumped in and out based on the accelearation of the vehicle. SO, if you are low on coolant and the sensor isn't telling you because it is probably broken/ old, it will not work properly. double check your coolant level. Also when you just sit in parking lot and expect coolant to swim around on it's own, it wasn't designed to do that. That is why upon acceleration , heater core will have ability to do it's job because upon accelerated it's designed to go through heater cored after being around the block. Coolant doesn't heat itself and so the water pump has a job to do. as long as the pulley driven device is doing it's job, the coolant can stay warm and take turns in heater core. good luck