ryanza said:
Here is a very mildly educated guess at what might happen re water cleaning carbon in an engine.
Steam reacts with carbon to form synthesis gas, this is part of the process to produce feedstock for a Fischer–Tropsch plant. It might be that the fine mist of water turns to steam on contact with the hot internal parts of the engine and that steam then reacts with the available carbon.
Pouring water, out of a cup, jug or hose into an engine is probably not the best way to try test this process.
Here's the simple analysis,compare expansion coefficient of carbon to the metal whether it's aluminum or carbon alloy. Carbon doesn't have an expansion factor relative to metals. Where the smallest amount of contraction can cause the carbon to simply break loose. This has been used in other processes.
There might be other dynamics as well, but induce a sudden temperature drop is very powerful. Then there's probably the expansion factor that might be induced by cold water turning to steam and under compression has an effect. What I do knows is, I sunk an Oldsmobile 455 and after removing water from crankcase and cylinders and putting new oil in, etc., the engine purred like a kitten. Inducing a thermal shock is so obvious.
An engine is regulated to run around 200 degrees in general. Some will block off coolant flow, then pour cold water in the intake. Which it can also cause cast iron to say "break". Cylinder gaskets, valves have to be considered too.
Just like dropping the exhaust pipe from the manifold removed so much back pressure that the Old 98, 73 model laid two rubber streaks about 10'ling, then a single streak much longer. Which without the pipe, letting off the gas can allow atmospheric temperature back up into the ports and warp valves, though it didn't happen. There's always what can happen.
Again one has to compare what's relative. If cylinder temperatures drop suddenly, the metal can contract where carbon doesn't . Say like aluminum, with a 200 degree change in temperature, shrinkage or expansion can be 2 or three thousandths. Again there's the skin or surface temperature that suddenly changed that doesn't translate into complete temperature affect of an entire body.
Lol, it's just like if you want to test and see if you have any spirits around you, yo, never mind, I won't tell you this. Someone might try then get concerned.