Is draining the coolant when installing a engine block heater absolutely necessary? In the instructions it says to drain after you remove the plug. If the plug is already removed why would you drain the coolant just screw in the heater and your all good, right?. Then top off for the small amount that spilled out when removing the plug? Any thoughts? Gonna install tomorrow and its gonna be 30 out and wanna do this as quick as possible! Thanks.