I will tell you what i did with mine with very good results. I pressure washed my engine several times and nothing bad happened.
What needs to be removed:
Remove:
1. battery connection
2. engine cover. (1.6 16v)
3. air filter box and intake hose
4. ignition coil
What needs to be covered:
Cover:
1. throttle body
2. breather hose end
3. the open part of the control box. It's the point where you removed the intake hose from.
4. the electrical connection of your coilpack
5. spark plug wire ends.
6. cover very well the area around the spark plug holes. You don't want any water to go there. Use your imagination. I used a thick and wide tape all around the spark plug wire guide and on top of it. (its the black plastic thing where the wires are)
I would not recommend to completely remove wires from spark plugs because they provide a good amount of seal to some degree, even though water can still go there if you insist too much while washing the camshaft cover.
7. fuses on top of the battery
Now you're ready to go. I would strongly recommend to get this product:
http://www.cleanyourcar.co.uk/index.php ... ductId=191
It does an AMAZING job! You just spray, leave it there for a few seconds to loosen up any dirt and grime and then finish up with a tooth brash and water to wash it off.
I used it to clean almost everything under the car, arches, suspension and the whole engine. It's the best.
It is important to be very careful with the pressure washer. Don't use maximum pressure, keep some distance from the bay and do not insist on the same area for more than a few secs. If you don't feel confident use a garden hose.
That's all. Good luck if you decide to do it.