Q: Why would fluid continuously flows out from the reservoir as soon as the car heats up?

The cap on a car radiator is designed to withstand internal cooling system pressure, as the system heats up, of 16 to 20 psi (pounds per square inch) depending on the particular vehicle. If your cap has failed, and thus is opening to the overflow reservoir too early, at a less than design pressure, the coolant will flow out of the cooling system as the car is running and the car may overheat as well. YourMechanic does offer radiator pressure cap testing and related diagnostic services that will confirm for you if a faulty cap, or another issue, is the cause of the overflow problem you are experiencing. Please let us know how we can assist you further in resolving this.