Hi nick, im a professional Valeter + Detailer. I've been doing it for 14 hers and I can help out here.
Start by giving the engine a coat of TFR truck wash, allow to soak in for two minutes.
Please remember be for pressure washing any engine to turn it on, avoid exposed electrics and ECU if possible.
Rinse with a pressure washer to remove all dirt, leave the engine on to allow access water to drip off and shaken around off the engine an what not.
Now over the years I've picked up a few tips and I would like to share one with you here. If you have access to a compressor and air line with a blower then give this a go.
Take a blower to the engine bay (don't matter if it's running or not) and have a clean micro-fibre rag handy. Blow away the water off the engine so it's nice and dry.
There's a piece of equipment know as a "tornado" it's an expensive piece of kit at £200 but it's the best £200 you'll spend, it can be used for cleaning seats, carpets, engine bays + a range of more things. If you have access to one then try this.
Fill it full with plain old paint thinners. Use in a be related area as it way quite misty. Apply a coat of thinners to the engine bay and allow to dry, don't afraid to be shy with it, you should get something like this:
Please feel free to drop a like on my page on FB, I serve the South Wales Cardiff and surrounding areas but I'm always about I offer advice :).
https://www.facebook.com/ValeAutoDetailing