I have always used the Gunk product. I warm up the engine, spray it... let it set for about 15 minutes and then go to my local high pressure car wash place, open the hood and blast away.
In the above statement, there are two things you are not supposed to do:
1) Run the car after spraying with Gunk.
2) Wash the engine in the car wash.
And I never cover anything. Everything under the hood is designed to get wet. In fact I often open the hood and give it a bath when I am washing the outside of the car. I do like to drive it a while afterwards to dry it out.
1992 green ssei. Tan leather, moonroof, power everything. 161k miles
Goodyear RSA Plus 225/60/16 VR on chromed factory crosslace rims. McGard chrome lugs.
Audio system: CD player w/equalizer, subwoofer (all stock).
Delco premium wires, NGK TR55 plugs, K&N stock style air filter. Ceramic brake pads. New rear brake tubing and proportioning valves. New wheel cylinders, front rotors and calipers. New front wheel bearings. Replaced crank sensor and harmonic balancer. Rebuilt instrument cluster. Cleaned A/C evaporator. Valve cover gaskets. Alternator. Front stabilizer links. Replaced all power steering lines and transmission lines. Cleaned MAF and IAC. New door appliques. Rebuilt steering gear. CERAMIC brake pads, I love them. I am listing all this in case someone needs advice on any of these repairs.