this is how I clean a very dirty engine... it's may be not the right or
elegant way but quite satisfactory for me..

- if the van is parked on a driveway, lay large cardboard sheets (from
shipping boxes) under the engine.
- spray WD40 on electrical parts (junction boxes, distributors etc.). Wrap
them with plastic wrap (the kind for food).
- spray the gummy areas with foamy engine cleaner or Purple engine cleaner.
- go over everywhere once with a stiff brush and mineral spirit and let it
sit for a few hours, I let it sit over night.
- next day (or after the waiting time) go over every where once more with a
stiff brush and mineral spirit and let it sit for one hour.
- use a garden spray bottle filled with hot/warm water, pump it up and
spray the whole engine, insisting on the dirtiest area. Or use a garden
hose connected to a hot warm water source. Do not use pressure  washer.
- Once cleaned, go over the whole engine with WD40 and let dry for about 1
hour.
- remove the plastic wraps and warm up the engine.

you're done  and now you have a clean engine.