When I did my engine bay it was horrible, just thick caked on crap. I sprayed it with oven cleaner then wire wheeled it. Then degreaser and wire wheeled it again. Then I scrubbed it all down with a really soapy mix using Ajax dish soap with bleach and stiff bristled brushes. Then sprayed with brake kleen right before painting. It worked awesome and was cheap. Took me a full day to really get my whole bay spotless, nooks and crannies included.

Roe