So my cars engine is very oily, and has grease and crud everywhere, especially towards the bottom.

 

id like to clean it. Firstly is this a good idea, and secondly how would I go about it? I’ve seen degreaser in the car parts shop but it can’t be as simple as put that on then power wash it off!

Try auto glym engine degreaser. It’s a spray on citrus degreaser. Allow it to dwell without drying the rinse off. Some areas will need a second application and agitate with a brush. 

When using a power washer in the engine bay, avoid electrics or cover them the avoid problems. 

Nice and greasy and not corroded,  so just think on when you clean it you need to protect again the alloy, steel and paint from oxidising and corroding / rust.

 

Power washed engines can look good for a while or like some of the horror shows Kerb Side Autos sell.

Then there is does your diesel / petrol still run OK, or will that depend on taking care with what you do with H20 and pressure and preparing the engine compartment.

For god's sake don't power wash the engine unless you want to start a whole load of electrical problems!

Never had any issues with mine after I power washed it. It will dry itself out quicker anyway because of the heat off the engine 

We pressure wash cars off all the time at work to locate oil leaks and have never caused a problem, avoid electrics obviously.

Put the degreaser on while the engine is still warm, power wash off avoiding obvious electrical items, people get precious about it but when you drive through standing water your engine is drenched, you just don't see it happen.

 

If it won't start or run properly afterwards just leave it to dry out with the bonnet up. It'll be fine.