I just cover the coil pack, airfilter, battery, and HID ballasts with plastic bags. On a cool engine, hit everything with lots of undiluted Simple Green. Scrub any tough spots, then rinse off with a hose with no sprayer on the end (just normal rinse pressure, nice and gentle). Engine will look brand new!
Simple Green not watered down should be plenty strong. I just used it on another car that in 10 years had never had its engine cleaned. After a good coat of S.G., everything rinsed right off.
Pull off the bags, start it up and let it dry.
