It is an
old idea that is used sometimes on engines to suppress knock. It has been generally shown to improve mileage a small amount. With water injection you can increase the compression ratio without knocking and advance the timing a little bit. I am of opinion that using excess heat to turn the water into steam thereby increasing the cylinder pressure is where the gain in mileage comes from.
I ran it before on my first Metro and I gained like 3-4mpg when it was working properly. The problem with the setups I tried and the commonly used
fish tank gang valve setup is that it never stays at the proper ratio unless you fiddle with it before every drive. That is a huge hassle and not worth it. I always wanted to use a fuel injector setup sized at 10% the flow of the stock injector and just let the computer manage it. But I don't think a fuel injector would live long running straight tap water.
The nice thing about that pump listed in the first post is it can pump any fluid without having to interact with the fluid so it won't care about pumping water for a long time. I just have to get the speed of it right and I should have a more or less exact ratio all the time. The pump came in today but my variable power resistor didn't show up so I won't get to hook it up and test it till Monday most likely.