I have always been super scared in cleaning my engine bay area. I just wiped it with water with my 3 year old 335i and thats it and left some grime around.
Anyway now that I've got my M3, the engine bay looks quite a bit more filthy so I finally got a autoglym engine cleaner, sprayed all over and brushed it and rinsed it with water, not from a power jet. Just gentle spray water from the main hose (but diffused).
Anyway all clean and all seems good the first time.
Did it again recently cause there was still some crap around.
HOWEVER, there was some fizzing, buzzing sound coming from the left front hand side and some smokes was coming up!
Absolutely kacked myself, but luckily it stopped after a while and the buzzing sound went away.
I've searched and searched the forum before I went ahead and everyone seem to say its okay to rinse with water but that was a scary experience man.