I spray my engine and everything under the hood with Simple Green, but only leave it on for about 5 minutes before washing everything off completley. I wash until there is no soap or suds left. So far no problems. I also spray my grill, bumpers, wheel well liners and wheels before washing. Removes bugs from front very good and leaves the black plastic very clean and a like new finish. I will spray the wheel liners, wheels and tire on the front, go to the back and spray then go back and wash and rinse the front while the back is soaking then wash it down. The wheel well liners look new and so far I have no problems, but don't leave Simple Green on to long.
Ron