Gunk Spray Engine bay cleaner is good stuff for that, Make sure you buy 3-5 bottles on the high side for diesel but it does work very well. For normal Gas engines (that arnt a grime fest) I normally get like Orange based concentrated cleaner at home depot and then use one of those garden sprayers that mixes whatever you put in it at a 1:5 ratio or whatever you choose and then I go to town on the engine bay.
I really like using hose attached dilution sprayer because it covers every little spot in engine bay and then you can use like pressure washer for final rinse.
Edited by Shata (09/05/17 10:52 PM)
15 Chevy Silverado 1500 /w L83 5.3
PP 0-20 /6k OCI (warranty)
Amsoil bmk23 /w Fram xg8a, eabp90