Ask 100 detailers how to clean an engine and you will get 100 different answers. It is just one area where everyone has their own technique and beliefs, there is no simple tried and true and well agreed upon method.
The products and technique are really not the important part, but some common sense is needed. While directly spraying the engine with product and a garden hose (not pressure washer) will in most cases not cause any harm, it takes 2 - 5 minutes to protect your work and is a very small price to pay to protect your customers or your own vehicle. You might know your own vehicles well and know what needs to be protected (if anything) and what does not. But imagine working on a customers car, and when you finish the job the engine will not start because the alternator is waterlogged. You just gave them a free detail, and possibly need to protect yourself in court when the dealer deny's warranty service. Most vehicles do not need anything to be covered, but it takes no time to do and should always be done.
When I worked at Lexus, we would always use a low power pressure washer to spray down the engine with no concern, until, a late 80's early 90's mazda pickup failed to start, and then a nissan 300zx failed to start. the pickup was because of water in the coils, the z was from water in the alternator. Lesson learned.
Thejoyofdriving: Ignorance will not protect you forever, you now have 3 reports of cars failing to start, it does happen.
