On stuff that has build up of junk I've used CLR to break down the junk and then flush it out.

 

I put some teflon tape on the plug threads and turned the plugs in hard enough so they can't fall off. I pressure tested the block with air to make sure there are no leaks. Sprayed soapy water around all ports and mating points to make sure. Found indeed a tiny leak at the temperature sensor on the block.