I've scrubbed both engine bays and engines with an 1800 PSI Karcher and had no issues. I do, however, tape up or otherwise protect ALL possible openings into the engine or anything else that might get wet (like the PS pump, etc.) before I spray. One time I had a pro cleaning the driveway and our house (we get algae growing on the house from time to time) and had him wash Mutt's engine bay (no engine in there at the time) with his high pressure commercial unit and it REALLY cleaned the snot out of everything. I don't know how many PSI he was packing but it was a LOT more than I have.

Before any of the above I soak the heck out of whatever is super grubby and sometimes even use the caustic Easy-Off oven cleaner which seems to do the best job out of anything I've tried. Be aware that it'll discolor raw aluminum to a dull grey color so cover it if you want to make it shiny some day.

My experience.....

Dan