The last time I cleaned my engine I used Purple Power. I was a bit worried about it eating painted surfaces because it warns that with full strength applications it WILL strip paint off. I diluted it probably half and half with water in a spray bottle and sprayed my entire engine bay down. Including the alternator. Then I immediately pressure washed the whole engine bay. Looks new. It's been 6 months or so and no problems. Still very clean.

I used to wash cars at the lincoln dealership and both myself and the detailer pressure washed every engine in every used car that came through that place and they always looked brand new when finished. First we sprayed them down with an industrial strength degreaser that I wish I could buy in the store and let it sit for 10 mins or so and then blasted away. Never covered anything, never had a single problem. The engine bay is designed with the idea that things might get wet once in a while.