How did you detail your engine to look like that? It always scares the hell out of me to get any water under there.
I don't use a hose or pressure washer under the hood or WD40. WD40 collects too much dirt and creates a nasty film on everything.
I clean the engine bay last so I don't contaminate my wash bucket with engine bay grease and grime. There is usually residual soap and water that has leaked down in the bay that I will use to spread about.
1.) Use a soft to medium bristle paint brush to loosen dirt debris. Do this first before adding excess water so you don't spread dirt around making a mud and scratching everything. You have to remember that the paint in the bay is delicate, not very thick, no clear and subjected to high heat; I don't like to use harsh chemicals or stiff bristle brushes as it will strip it off.
2.) I take a soapy damp rag(microfiber usually) that Ive dipped in my wash bucket and rung out, not dripping wet. Then go over everything metal and plastic with it. All the hoses and lines and inspect my vacuum lines. I don't usually go over the decals or vin tags with copious amounts of fluid or vigor so I preserve them. I don't want the water getting under it and removing the glue or rotting the paper, I don't even want the rag catching a corner and peeling it up.
3.) I then rinse the rag, ring it out, dampen it with clean water and rinse off everything. At this point the rag is pretty dry and fairly clean so I do all the plastic trims, inner fenders, hood, radiator support, battery etc. Then I might come back with a shop towel if there are puddles, spots or streaks.
4.) also its a great time to inspect your fluids as some usually drip out or you may have to add some, and will promptly be cleaned up, look for leaves and debris that get stuck in the cowl.
TIP 1: If you are concerned about anything getting wet use tin foil to wrap and cover up anything sensitive like fuse trays, computer components, coil, distributor etc.
Tip 2: I use a makita battery powered leaf blower (milwaukee also makes one if you have their line) to dry my car so I don't get water spots and also blow off the engine bay in case water puddled around the intake or fenders to help prevent rust. A compressor would work in the bay, as well as lug nuts under seals and trim, mirrors, door handles, gas door etc. Its just more precise when I want to blast everything.
Hope that helps,
Shane