Simple Green works great. My '87 240 had never been cleaned under the hood before I went at it last summer. On the exhaust side down near the firewall, the oil and dirt was so thick that I needed a stiff plastic brush and several applications of SG to cut through it. I was amazed at how white the paint was underneath, but I guess it had been well preserved! The under side of the hood was also unpleasant, with a build up of crap that sprays off the belts. Start near the latch and work your way down because the water/cleaner runs down and drags the dirt with it. The hardest part is probably the drivers side with all of the wires and hoses. I just sprayed the areas I could get to, and used SG on a rag to reach areas that I didn't want to spray with the hose. I used a hose nozzle that mists the water, so it was easy to control the spray.

I spent a few hours just cleaning the engine bay including the fan shroud, airbox, and big hoses, and a couple hours on the underside of the hood alone. Detailing all the wires and hoses would probably take a long time, and I still haven't cleaned the engine in my car. It's a lot of work, but the end result is great.
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