George, order yourself a gallon or two of Spray Nine heavy duty degreaser from Amazon and find an empty spray bottle to use for applying the degreaser. Use the spray nine on your firewall and engine, and you can get most of that grime off and looking very fresh. I would suggest scraping some of the heavier deposits off the head before going at it with the degreaser. You'll be amazed at how clean your engine bay will look! Use a toothbrush or small dish cleaning/scrubbing brush to agitate after it has had a chance to soak in and then use a hand pump weed sprayer with hot water to wash it off. Be mindful of the aluminum intake, I personally have not had bad experience with the spray nine on aluminum, but I have had bad experience with the castrol superclean etching an intake before