Yup, it's perfectly fine to give the engine bay a scoosh with a pressure washer every now and then.
With Land Rover engines, you don't have to be particularly nervous about getting them wet...especially when you consider the wade height on the D5 is half way up the doors (900mm)...that's also half way up inside the engine compartment.
Even the airbox is sealed...you need to remove half a dozen screws to remove the lid for a filter change. It's also sealed with a rubber gasket.
Just don't use a high pressure direct jet, or adjust the nozzle to a fan spray when applying the foam and then rinsing... or use a garden hose to rinse it off. I haven't done mine on the D5 yet...but it certainly needs it with all the general muck and spray we have up here.
Just avoid directing a high pressure jet onto bits like electrical connectors and fuse/relay boxes you aren't sure about, like most of them under the bonnet, they're water and weather proof but not necessarily pressure washer proof...
But generally, cleaning the engine bay is a good practice. Makes it easier to spot oil leaks and the like
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