I usually get it a little wet then spray Simple Green. I let it set for a few minutes then blast if off. Just don't let the Simple Green dry on. They also make some special engine cleaners you can buy at your local auto parts store. I think Gunk has something called Engine Brite that we used to use at the dealership I worked at. Don't get to close to any hoses or electrical connections with a concentrated water blast. You can push water into them and then corrosion. I usually stand back a little ways but not so far that it doesn't clean away the crap.
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