I always have on every car/truck I've owned. Usually about every 5k-10k miles. I like to see a clean engine when I open the hood.

 

I wash the engine bay about 2-3 times a year. But I don't use high pressure spray just a garden hose to avoid casung problems to wiring, electrics, etc.

Usually I make up a plastic spray bottle of a couple of cap fulls of Lysol, a small squirt of dish-washing liquid, and hot water solution and spray it generously all over the engine and engine compartment - using most of a 24 oz bottle on the engine bay area.

I let that soak for 5 minutes and then gently rinse the engine bay area taking care not to force water into fuse boxes, wiring, air intake, etc. Then I wipe down all areas with a dry rag to get any stubborn dirt off. Put the plastic covers back on and immediately take it for a 5-10 minute drive to make sure the engine bay gets fully dry.

Then I wash the exterior of the car to get any over spray and dirt that may have been blasted out by the hose onto the exterior.

I think it makes the engine bay components (belts, hoses, wiring) last longer when they are regularly cleaned. And I also think that when a mechanic works on your car they notice and appreciate the spotless engine bay, and maybe they might try a little harder to do a good job - just a theory. YMMV

 

i always used meguires wheel cleaner (purpelish/pink bottle) works amazing on colored aluminum and black plastic, i been using it on my motorcycle for about a year now. i just spray it on dry surface let it sit for few seconds then whipe it of and folow with your typical wash. that stuff makes black plastic peices black again it gets all that white stuff off black plastic and aluminum

 

i usually clean both engine bay and under carriage too...what i do is jack it up put the car on jack stands then use simple green to hand wipe the the parts.