I find the best way to clean an engine bay is to use a Gunk product which is for cleaning concrete. It comes in a small red plastic container, and is available at Halfords. Place this in a spray bottle, spray all the engine, work it into the dirty bits. Leave for a few minutes, then pressure wash off. The alloy will come up like new, with the rest being clean. After this you must spray the engine bay & engine with WD40 or the like, or the alloy parts will go white. But the WD40 gives the engine & engine bay a heathly shine. Hope this helps, if you require any further info contact me at, michael.hutin@tesco.net