Degreaser works wonders, don't bother buying expensive products, my engine bay was cleaned with a couple of bottles of cheapo 99p degreaser, just spray it on (standard EP91 N/A engine so didn't really need to cover anything up), then just keep spraying more on and the dirt will just fall away. Then get the hose pipe on a very fine mist and rinse it off, it comes up really clean and that's without any effort! Just spraying the degreaser on then rinsing it off, and it barely cost anything! Once that's done start the engine and let all the water evaporate off.