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Use a paint brush to loosen surface dust, then vacuum it up with a shop vacuum. You may want a few different size brushes to help you reach into all the nooks and crannies. If you don't have a shop vacuum, you can blow the dirt and dust away with a leaf blower. Go over the entire engine with the brushes and vacuum. The intake sits at the top of the engine and is usually made from aluminum, which can get stained and pitted. Rub a metal wire brush over the entire intake and any other aluminum parts, like lettering plates, to clean them. Be careful not to rub any hoses or sensors with the metal wire brush. Alternatively, you could use a rotating wire wheel to clean aluminum parts. Spray the engine bay with a weak stream of warm water before applying any cleaner to remove loose dirt and grime. Avoid spraying the covered electrical parts. Rinsing the bay first helps the cleaner to spread evenly and reduces spotting. Make sure you don't use cold water, which could crack the block. Diluting the cleaner or degreaser provides you with a more even clean. Use 1 part water and 1 part cleaner. Spray the entire engine bay except for the covered electrical parts. Pay special attention to the firewall, fluid containers, hoses, and caps, which tend to build up grime. Products like Simple Green and Purple Power Degreaser work well for detailing engine bays. You don't want to use a high-pressure sprayer, or water may be forced into places it shouldn't go. Use a weak stream of warm water to rinse the grease and grime off the engine. If the engine bay still looks pretty dirty, spray more diluted degreaser, let it soak for 5 to 10 minutes, then rinse it away.
Agitate surface dust with a brush and vacuum it up. Scrub aluminum parts with a metal wire brush. Wet the engine bay with a weak stream of warm water. Spray diluted degreaser and let it soak for 5 to 10 minutes. Rinse away the degreaser with a weak stream of warm water.