This is an old detailer's trick that I've been using for years. First I spray the engine bay, top to bottom, with Gunk engine degreaser. If the engine is really dirty I will use a plastic bristle brush like you use for tire rims to scrub the nastier parts. I then follow up with a pressure washer. I've been steam cleaning and pressure washing engines for years and never had a problem, but them I use a little common sense as to where I blast high pressure water. Your mileage may vary. Anyways, once the engine is clean I use a leaf blower to dry the engine off. Next spray down the engine compartment with foaming tire shine and close the hood. Come back and take a look in about 2 hours - engine will look as good as the day it left the dealership.
FYI, this is an old engine detailing trick that has been around for years. Google "engine bay detailing tire spray" if you don't believe it. I'm sure some internet "sage" will chime in with how they ruined an engine using a pressure washer but if you can't tell the PCV valve from the ECU then maybe you should leave everything under the hood to your dealer.