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Typically when I wash my engine I spray a cleaner such as Simple Green on everything. Wait 5 minutes, then lightly spray it off. I avoid getting a lot of water around the air filter, battery, fuses, alternator, and any other electrical wiring.
The most important thing to remember is to only spray water on a COLD engine. Spraying water on a hot engine may crack components.
I also wait for everything to dry before starting the engine.
Leaf blowers come in handy for washing the car too. My brother runs a commercial landscaping company and he always takes his backpack blowers and blows the trucks off after washing them, to avoid water spots.