I bought something called "Castrol Super Clean" at my local Wally World last week, and I cleaned by engine bay with it - 7 years worth of .. err.. whatever was stuck there..
The gunk and grime stuck under the hood mostly came off on contact with the cleaner, the rest came off when I hosed it down for the rinse. Surprising results!
Now I have to clean the rest of the engine itself, as it looks like .. well... looks bad.