My daughter has used Coca Cola on a rag to clean off the brake dust and crud off the very bad looking chrome rims on her truck. It worked very well, took the bad stuff right off, clean off with water and they were sparkling again. Heard that it cleans up old tarnished pennies also. I personally wouldn't use it under the hood of my car, (that was probably a joke anyway, right??!) I don't see it removing oily gunk anyway.
I had read on the board about several people recommending Simple Green full strength, that stuff is great - when you are done with it on the car, bring it on inside works wonderfully on jobs in the house also.
I would also second about NEVER USING OVEN CLEANER UNDER THE HOOD OF THE CAR! That stuff is nasty, hated using it on an oven - very caustic, terrible smelling.