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I've been hosing down my engines, both on my '01 and now on my '05, every time I wash my truck. I don't cover anything and haven't seen any ill effects, except for a saggy hood liner on the '01. Avoid the liner and you shouldn't have a problem.
Don't know about the ill effect of Simple Green on your engine, but we quit using it on or around USN Jets because it caused corrosion and etched the windscreen and canopy's.
What I've found that works great is Gunk's Foamy Engine Brite. With the engine warm, not hot, spray everything down under the hood. Let it sit a while then hose it off. I do this at a manual wand car wash. While your washing the rest of the vehicle, let the foamy brite do it's thing, then when it comes to rinse time, do under the hood. Use some common sense where not to rinse under high pressure. This will clean the engine and compartment up nice. For detailing it to look like new, mist some armor all or similar product all over the engine and compartment and be on your way. When you look under the hood again, you'd think a dealer or body shop just detailed it. Looks great. I've done this for years and it works great with no ill side affects.