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Oven cleaner works about as good as anything I've tried. Just avoid your wiring and plastic parts and rinse it down really well afterwards. You may even want to cover your alternator and other electronics with a plastic bag as best you can.
i work in the detail department at a car dealership in illinois. when we clean a car for the customer we leave the car running, open the hood and use a pressure washer to spray off the engine. if its really dirty we use a basic all purpose cleaner that we just spray on the engine. it works really well.
x3 on the simple green... once you have it sprayed down, take it to a car wash and hose it down with the engine degreser setting... it'll come out nice & clean. be careful not to get any open air/breather filters wet.
Just spray the heck out of it with engine degreaser, let it soak for 5 minues, and then pressure wash. No need to cover the electronic. Repeat as many times as nescessary.
If you don't have a pressure washer, those coin operated spray places work fine.
I have been cleaning dirty car engines for years. I warm up the engine a little if it's cold, or wait a half hour if it's hot. I will start under the vehicle, and spray Gumout Steam Premium degreaser around the oil pan, transmission, and base of the engine block. Then stand up and spray the degreaser from above on the firewall, brake master cylinder, and anywhere it's greasy. Let sit for 15 min, then hose off with a hose or pressure washer. Usually I will repeat with Simple Green, then start the engine and idle it awhile to dry it. The smell of the degreaser will usually linger for a day or two, but the results are fantastic.