And here i was just drowning the entire engine with degreaser and hosing it off.... tbh i've never had a problem doing this unless there was something already wrong, crippled a vl or 2 doing this but the insulation and rubber boot around the cas plug were shot (learnt the hardway).
Have never killed a v6 doing this, upset the lpg controller once, found a faulty coil and found a dodgy spark plug lead boot.... but have never actually killed anything using a pressure washer.
Don't really need to cover anything, if using a garden hose then there's little to no chance (unless plugs are broken and wiring insulation is cracked) of doing any harm.
The plugs and connections used are IPX rated to keep water out, once again... unless something is not upto scratch and deteriorated you should need to cover anything.
If using a steam cleaner/pressure washer/car wash then the simple rule of stay away from anything with wires connected to it will suffice.