There's no harm with washing a COLD engine. Just make sure you aren't afraid of rust and oil stains on the driveway when you are done. And when you are done, let it dry for about 2 hours before you start it. If your wires and hoses are in good shape, you should not get water in spots where it don't belong. No need to worry. Like has been stated many times here already, hot metal and cold water do not mix. You want a real demonstration of this drop an ice cube in a cup of hot water or coffee. See what happens.