You can pressure wash the engine, but the prep work is very important. Take some plastic bags and cover:
- battery terminals
- alternator
- fuse and relay boxes
- coil packs
- any other visible wire connectors
After covering, tape up the bags with painter's tape, also wrap any wire connectors with painter's tape. Get the Frog tape, because it's a loud, obnoxious green, and it'll make it easier to identify when you have to remove the taped up areas. When it's time to remove the plastic bags, be extra careful, especially around coil packs. Besure to mop up the water on the bags with a rag, or just be very careful removing the bag without spilling any water on it.
Be gentle when using a pressure washer. Don't spray in one specific area for a long period of time. You're not removing the grime by using the pressure. You remove that by using degreaser and elbow grease. the pressure washer should be used to rinse off any gunk and soap residues.
It's not as scary as everyone makes it out to be. You just have to prep the engine bay before you start spraying. The result is very rewarding. It won't make your engine run any better, but it is fun to stare at a clean engine bay.