A trick I have used in the past was to fill the cooling system with water and toilet bowl cleaner. This of course is on a running engine, although you could probably use it in a pressure washer. Once the engine is brought up to temperature I would just let it run a little above idle for an hour or so, on a really dirty block you could run it several times allowing it to cool between runs. Afterward you just remove the thermostat and run a garden hose into the radiator and run the engine at idle with the drain valves open until the water comes out clean. I have used this on several engines with good results, it does leave the shop smelling "minty fresh".
