First off you must do this with a cool engine ... cover the intake filter and the ENTIRE valve cover. Then just spray some Simple Green over the bay and let it work it. Problem areas hit up with brushes or sponges. Rinse off. Detail ... blah blah blah ..
Everyone knows a soaking wet filter is bad news but I say the valve cover for a few reasons:
1. Coil on plug -- you dont want the coil packs getting water logged.
and the most important...
2. The water can work its way down into the spark plug wells and will cause the plug to ground out as it fires. The car will basically not run, and if it does, will run REALLY bad. To fix the problem you need to pull the coil packs off ... dry out the wells as much as you possibly can and then change your spark plugs. You run the risk of the water getting in with a regular hose pressure much more than you do with a high pressure washer since the HP will blow most of the water away as you use it. Either way its better to be safe than sorry. You can later use instant detailer on the valve cover to clean it up if necessary.
For years we cleaned my bf's engine in his 99 Cougar without covering ANYTHING ... we got my car and cleaned the engine bay the exact same way and guess what --- water in the plug wells.
Pe@ce