Hello Mustangjs, Yes that the Boyle's law applies very well to the radiator problem. The cap of radiator is from original manufacturer and it closes the radiator very well. When the gauge of the temperature is on normal level (horizontal), I can open it without any problem but if the gauge indicates hot, I will never open it until the enginee cools down. Thank you for your comments.<P>Gord, Perfect explanation. The prestone marketed in Peru is written in spanish and it is already mixed with water desionized, refrigerant and coolant and it is ready to pour into the radiator without any other mixing. It has no teardrop shape but a rectangle shape. Many thanks<P>Rodolfo<P> [img]images/icons/wink.gif" border="0[/img]