What I have found works well for cleaning under the hood and around the engine is a very diluted mixture of water and "marine clean". Mix about 2-3 cap fulls of cleaner to a gallon of water. Using just a garden hose, wet down the entire engine bay, underside of the hood, ect. Being sure not to spray too much at the alternator or directly into the air intake/air filter. Then using either a spray bottle or pump sprayer soak everything that is dirty in the cleaning solution. Let it sit for 5-10 mins but don't let the cleaner dry onto the surfaces. Then hose off the cleaner. There may be some trouble areas that need a scrub but the "marine clean" is good stuff and cleans really well.