I recently used Simple Green Degreaser (Bought at Pep Boys) on my engine and also a scrub brush, and the next day when it dried I seen the biggest difference ever. It is great stuff. I once also used Gunk's spray on degreaser, not as effective. Just make sure you don't spray an air filter link the k&n filtercharger with it, or water, that'll be bad, I cover it with a plastic garbage bag and rubber bands. As far as the electronics, don't overspray with degreaser or water. After this let it dry with the hood open (for more ventilation) for about 5-6 hours. If not, well I burnt out a throttle position sensor b/c I didn't let it dry, I was also using a pressure wash. I recommend just using your hose.
There are many "purpose-built" engine degreasers, and some work
o.k....however, the best all purpose super strong cleaner/degrease I've used yet it
Castrol Super Clean. It's sold in white bottles with purple and orange printing. Stuff is
SUPER STRONG !!! It will actually take clear coat off certain things if you let it sit
long enough ! (my dad's been soaking the old floor tiles here at work, and it's stripping
the old floor wax from 20 years ago !
Anyhow, on most of the motors I clean, I spray this stuff onto it, (careful not to inhale it... catch one whiff, and you'll cringe from it's strength...) let it sit for no longer than 10 minutes max, and hose off. It'll remove valve cover oxidation/grease, scum on the radiator, and all excess dirt and grit from the engine and hood. As with all motors, I make sure I air blow them off with compressed air after, as you can cause ignition problems on some vehicles. (Honda boy's V-Tec motor is SUPER sensitive to water, and their spark plug boots don't seal as well as ours, and water collects in the spark plug area.) If you have an external air filter, try to stay away from soaking it down, you can even stuff a plastic bag over it to protect from excess water. Also don't soak down electronic areas like the coil, distributor etc.I fire up the motors after pressure washing them, and blow them off with compressed air. Only drawback to using Super Clean, is that it'll strip wax instantly, so your fenders and hood may need rewaxing.
I wash my engine bay down several times a year, and haven't encountered a single problem yet.
A word of warning
I always gunk the engine, but one day the transmission stopped shifting up totally the warning code was 16 flashes. the Nissan dealer could not figure it out at all and of course it shifted fine for them. I found that this problem is the "throttle valve switch "the connector is facing the front of the car just right of the throttle body. I opened it up , and sure enough the connector was filled with rust and water. dry it out, and seal the connector boot with black silicone and this will never happen again.