I only use a garden hose. Be careful if you use a degreaser. It can discolor some of the underhood and engine components. A soft bristle brush works wonders. You do not need high pressure. Stay away from electrical connections. After washing, run the engine to warm it up and it will dry most of the water.
I clean them with a Steam Jenny all the time!! I use very little cleaners other than 3 oz Lemon Joy and 1/2 oz bleach in 3 gal water in a yard sprayer!!! "Cold Engine" Soak it down with the solution, let it soak for 10 min and hit it with steam!!! 230-250 deg water will clean more than you think by it's self!!!