I would give it try with dry ice cleaning. Call a dry ice blasting company and ask if they offer the cleaning as well. Here its 150€ for the engine and isnide of the car. Best thing is, no water is used which means no harm to electrical components.
Works for every part of the car.
Look here
Oh yea, switch of the volume. At 1:18 they show the engine.
Thank you for bringing this to our attention, a very interesting option that I'd have never known about if not for your post.
Took me less than 30min to find 3 companies in my state that offer DryIce Blasting for interior + engine bay detailing... one of them even does the undercarriage and "spot treatments" on the body for rust removal (not an issue for me right now, but good to know for the future).


