I have what I assume is a coolant leak, but I cannot find out where the link is occuring. I read other posts that indicated if the leak was pooling under the floor mats on either the driver or passenger side then it is probably a heater core issue, but I don't have any pooling inside the cabin. First, if you only look after it has disappeared, there is no coolant left to be leaking Second, coolant will often leak only under pressure, when it expands from being hot. This often happens when... Dec 11, 2014 · Trapped air, water-pump not driven/impeller sheared off - due to freezing of coolant, thermostat stuck closed, blockage in the cooling system (rad weld, for example). It really depends on how quickly the pressurization occurs. Stuck thermostat will take a while to pressurize the system (as it warms up). At around the same time radiators were getting smaller and the tanks on them were disappearing, the need for higher and higher coolant temperatures – for reasons of efficiency, also occurred. Because of these two factors, an additional expansion tank concept was born.