I´m cleaning engine compartment 1-2 times a year on my 964T '91, just spraying on "any kind" of grease removing automotive product, let it stay for a short while and flushing with high pressure washer.
Normally I don't cover anything and so far nerver run in to problems. However it's a good advice to cover powersteering and brakefluid vent holes and avoid spraying electrical parts directly or cover them since sooner or later it might cause problems (especially the high pressure water has a tendency to eat through stuff).

Also a good idea to avoid water getting into the brake fluid and clutch reservoir if you have one. Small bit of tape over small breather hole in the cap will suffice.

Take care,
John H from Downunder

The engine should be cool or cold. Never spray water on a warm/hot engine.
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john,,1985, 245:GLTi 235k mi..so.california

I love this stuff, Castrol Part cleaner, Purple stuff and it's biodegradable. It it crossive, and you don't want it on your hands. Soak the engine, and go over nasty stuff with a nylon brush. Rinse with water.

I have heard it said to bag the distributor and the AMM. I worked at a car detailing place and they never bagged anything, I can not remember us ever having a problem on many makes/models, perhaps a few hard starts.
If still cautious try not to directly spray degreaser or high pressure water on the distributor, amm, ignition module, throttle position switch, (ie anything electronic or working as a switch/contact), general wiring and engine sensors seem to hold up OK.
Some places put dye in the oil to find leaks (perhaps with UV light)
Have fun.
Karl

What I do is to get a can or 2 of Gunk spray engine cleaner and a bunch of
quarters. I drive over to the do-it-yourself car wash and spray down the
grungiest parts, sometimes going over them twice if they are really bad.
Let it sit for 10 minutes or so.
Then put in the quarters and select the strongest detergent the machine
has. (some say "engine cleaner") Wash off the gunk and grease, then go
back with the clear water rinse and wash off the detergent. As soon as
you are done, start your engine and let it run until at least the parts that
get hot are dry. It may take several washer cycles to finish everything.

Before doing this you may wish to put a plastic bag over certain electronic
parts. I haven't had much of a problem with this because most of my
electronic parts don't get that slimy. Good plug wires do OK but old ones
will give you heartburn until they are really dry.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma