I plan on cleaning mine. I plan on using plastic bags to cover the alternator and airfilter and any other important items then using a cleaner like All Clean from chemical guys and spray over the engine bay with then agitate with a brush and finally pressure wash with a 40° nozzle so it's not a lot of pressure on one area. Mines pretty dirty as well. Pressure washing or hosing with a degreaser seems the most logical way and can get into crevices brushes cant.
I've killed a BPS and caused the angle gear to leak by being too aggressive with cleaning. If you're going to spray it down don't use high pressure. Get a cleaning agent that will do most of the work and spot clean other areas by hand. I know for fact that the plug wells and top valve cover will fill up with water if too much is sprayed on top, all other areas seem to handle water spray well.
I actually use a pressure washer over a hose. I'd rather use pressure than flood out the engine, for the Engine most degreasers or APC's work great. I personally use Nextsett W99 with an electric pressure washer, just mist it off after a little bit of brush agitation and your set.
Just like in a car wash, you have 2 methods for removing dirt.
- physical contact or
- loads of high pressure water
I flooded the distributor on my old chevy truck when I was 18, so I learned my lesson, less is more.
I've done this on several cars in the last few years, several Volvos, and the wife's Honda to clean off an oil leak. Obviously depends on what's going on, but this is what I have on hand:
Engine degreaser aerosol
Bucket of hot soapy water
On my S60, I didn't have any giant grease accumulations so didn't need to do any of the detail work. I more or less could use a wet sponge and rags to wipe and clean as needed and rinse with low volumes of low pressure hose water.
On my '72, I definitely had decades of grease to gently work out of the nooks & crannies. Again, little bit at a time, definitely didn't want to use high pressure water and destroy rubber or gaskets, this took me an evening to do a section at a time.
On the Honda, the entire backside of the block was coated in gunk. This I sprayed down with the aerosol and had to flood it to actually get it halfway clean.
Your mileage may vary, but I find a hand wash and using as little water as possible to be the safest move, then it's a simple wipe when you wash the car, even if it's infrequent.