The confines of an engine compartment never seemed to me the greatest place for using a pressure washer. I'd likely get more water on me than on the engine, and couldn't get the best angles with the nozzle. More importantly as Gerald points out, it'd be too easy to force water into some component that should never see it.
On the farm years ago I often used the Gunk method on oily dirt, and it worked well. In your case though I'd just wrap any electronics (mostly found around the top where road spray normally doesn't go) and the alternator, starter, and any obvious openings with plastic wrap or shrink wrap on small hand spools like I've found at hardware stores. Spray with Extreme Simple Green for Motorsports, let soak but keep damp, and hose off. Hot water as Bill mentions might be helpful, but that's up to you and how accessible the hot water is. It shouldn't be that critical for just the dusty engine you describe. I'd warm up the engine after removing the plastic wrap to hasten drying of any areas that don't normally see moisture.
My engine isn't all that dirty, as I've carefully just hosed it down several times over its 16000 miles so it's not accumulated. Stubborn spots with oily dirt earned a Simple Green soak first, but I never had to use wrap yet as I just kept away from water-sensitive components.
Joel
