If you plan to open the radiator cap when the engine is hot, keep in mind that antifreeze and pressure raise the boiling point of water.

This means that when you release the pressure, the water is above its boiling point, especially at some locations, such as in the cylinder head.

This super hot water can flash to steam and expand something like hundreds of volume, pushing hot water out toward you.

I have seen hot water spew 20 feet in the air when my neighbor removed the cap from a friends truck that was over heated, the blast did not hit him directly, but the rain coming down did burn his back as he ran away. The reason I saw the incident was because I was running toward him yelling for him to stop!

Manufacturers warn you against opening the radiator when the engine is hot - with a flaky temperature gauge, the water could be super hot.

As a firefighter, I saw about 2 dozen hot cars - we always kept them running and hosed down their radiators before shutting them off. One Dodge van was so hot it dieseled even when shut off - we had to spray C02 into the carb to shut it off.