That gunk sticks TIGHTLY! There's some kind of special Diesel engine grime that's tough to get off. I've had the best luck with the ultra-toxic Easy-Off oven cleaner - follow label directions for an extra grimey oven. Yes, it's toxic as He!! but it actually works. Scrub where you can (wear gloves and a face shield!) then hose it off with the power washer. It won't get it all but it'll get a lot.

Take anything you remove (say, the cam cover, oil pan, etc.) and to your local auto machine shop. They have a high pressure, high temp cleaner cabinet that they can put them in and get them cleaner than you can at home.

All of the above is well worth doing.

Dan