It would depend on how dirty your engine compartment is, but I 1st of all am a professional detailer and own my own company with 15 Yrs and 4400+ vehicles detailed. Anyway, I normally use Either Purple Power or Castrol Heavy duty degreaser, unless the grease is thick, then I use oil eater ( caution : oil eater is hot, and will etch [stain immediately ] aluminum ). If selling your vehicle, and thats why your cleaning the engine compartment, spray with the castrol or purple power, all over engine compartment and let sit for 10 minutes, brush any heavily greasey areas and reapply more spray after brushing while in your 10 minute window.
Rinse with strong stream of water ( I use a pressure washer, but stay off obvious electrical connections or around obvious places that will damage ){ do not use a steam cleaner as steam ruins computers }, immediately after rinsing entire engine bay , start the vehicle and let the engine dry your compartment.
Then if selling the vehicle , but not driving at all or very little......use an Oil base tire dressing ( clear "Mequires Hot Shine" works best ) and spray all over engine compartment and let dry.
If selling, but need to drive the vehicle and still want a nice finish on the black rubber engine compartment use leather conditioner on all the hoses, black plastic pieces, etc.
If your selling a classic car or the like and want the inside to be factory looking, use baby powder on the hoses and the plastic pieces.
note: Know that dressing applied to your compartment is a grease and dirt magnet , so apply according to the above application.
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