There was a very good post about this not too long ago, if you want to search it. I use hot water pressure washer and some degreaser, Then follow with a water based protectant. When I went to a follow up class a couple of moths ago, they were discussing engines and how to keep the whole bay clean. They discussed how car dealers like them done and how a normal cumtomers would want theirs. My understanding is that most engines are water resistant and that pressure washing at low psi is fine. I always wet it first, then let it semi-dry, then spray a degreaser on it, and rinse. I have never had a problem with any of the cars I have did. Remember to do it before you wash the car, and it usally takes about 10 mins to do it.