A cold water garden hose, dose just fine. Most car washes use a soap
that is high in acids. This can cause problems under the hood. Never use a
soap wand at a car wash under the hood.
If you want soap, get a safe engine cleaner spray from an auto parts
store (degreaser), if the engine is grimey. Then rise it off. As for
Dex-cool, any temp water is fine, and no soap is needed.