When I wash my engine I cover the alternator and the coil packs with plastic bags. If you have an exposed intake cover that too.

Pressure washing isn't necessary unless your engine is terrible. I usually spray the degreaser, let it sit, work it in with a brush and rinse with low pressure water so nothing gets knocked loose.

I barely even do that though, the engine has gotten washed as often as the outside of the car since day 1 so it's pretty much spotless.