I never use the engine cleaner sold at the parts shops. Any degreaser or all purpose cleaner will do fine and won't leave that nasty odor that 'Gunk" cleaners leave behind. Even though our battery sist in a closed area, I open that door, cover it with a plastic shopping bag, and replace the door. Unless you have added an open air filter like K&N, there is nothing else to be concerned about. I see people use a high pressure wand in the engine bay with no problems but I will only use the garden hose with a nozzle that allows for a moderate spray. You merely need to rinse, not attack the engine. I start byu wetting the underside of the hood, then apply cleaner, agitate w2ith a soft bristle or detailer's brush, then rinse. I do extra dirty parts multiple times until the entire underside and pad is clean . I use a blower to dry it as completely as possible.
Then I move to the outside edges of the hood and fender area following the same method. I wash the engine covers separately and leave them to dry on the ground.
Next I do the firewall, the inside of the grille, and the top of the engine. Lastly, I do the rest of the engine, being sure to lightly brush where I can reach including the intake hoses, fuel rails, reservoirs, etc. Then I blow dry the entire engine bay. Do NOT use a hair dryer! I am speaking of an actual air blower with filter but you could probably use a leaf blower. Finally, after the surfaces are dry, I spray a dressing liberally over everything. Then I use the air blower to spread it evenly thruout the entire bay.
This is the hood and engine bay of an 11 year old M35 with over 100k miles on the clock:
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what I use:
Chemical Guys Orange Degreaser:
https://www.amazon.com/dp/B00DZI39UW/re ... s9dHJ1ZQ==
7 pc brush assortment:
https://www.amazon.com/detailing-17inch ... 1&sr=8-104
AutoGlym vinyl / rubber dressing:
https://www.amazon.com/Autoglym-VRC500U ... 207&sr=8-1
Auto air blower:
https://www.amazon.com/Chemical-Guys-AC ... 195&sr=8-6