I cover my cone and away I go with the power washer. As for the chemicals that I use, i would say just the power washer, then if it starts getting really durty I will go get a can of gunk (the stronger one) and when it was really bad i used 2bottles of simple green (The big bottles) and about 4 roles of rags to clean it out. (thats when it was parked for the winter 2 years ago.
I never formally clean my engine bay, sometimes I use some Armorall when
I am already cleaning the car to clean the valve cover, stb, etc. I am just
lazy though, but the Armorall helps rub the dirt off and makes stuff
shine so meh, good enough.
I just cleaned my engine bay this weekend, scrub it with gunk, till its pretty clean
then spray teh whole engine bay and let the gunk sit in there for about 45minutes and powerwash it all out
it looks brand new, well except for the rust