I ran across several YouTubes that claim, and apparently demonstrate, that you can clean carbon deposits from a car engine by simply spraying or pouring (very slowly) water into the intake when the engine is fully warmed and running. I've never heard this from a reliable source and my reaction is that it is probably an urban legend, like removing rust with coca cola and aluminum foil. However, Eric The Car Guy, in the last video, is usually mainstream, so I'm not sure. The principle is alleged to be that, upon hitting the hot piston, the water flashes to steam and dislodges the carbon deposits. And in the second video it asserts the idea for this came from examination of engines whose head gaskets had failed: the pistons onto which coolant had leaked were weirdly clean while the protected pistons were encrusted with carbon. What is the real story on this?