Do you mean hot or cold? You would want to wash it with the motor off that way, if you have an aftermarket air intake, you dont suck in any water into the motor. The sooner you get the dirt and mud off the better. I've left my engine bay muddy for a few days and the mud ends up baking on everything making it harder to remove. If you go to a self service car wash, make sure you watch were you point the hose, that kind of pressure can mess up some of the wiring for the engine, making the dirt the least of you problems.