If you guys really want to be wackos, take a look at the "hard lines" for the A/C system. The parts that aren't covered in rubber are bare aluminum - they can be polished with some metal polish.

 

The only things I ever cover in my engine bay is the intake filter and the alternator. And I just use an old MF cloth.

The way I see it is, engines are designed to withstand some water. You get a lot of water in the engine bay when it rains...why do you think it gets so dirty?

I'm not spraying those parts directly so even if the cloth gets damp, it isn't that big of a deal.

 

I've cleaned probably a couple hundred engines when I detailed and never had any issues. Never covered anything except for an intake cone filter and always kept the engine running.

 

I ALWAYS pressure wash the engine and have been doing it for years with no problems. I actually just pressure washed it today to get rid of some of the salt that splashed into the engine bay