What works the best for that is some dishwashing soap. First remove the thermostat then put in about a tablespoon full. Run the engine for about 30 minutes. Now remove the upper radiator hose from the radiator side, get a garden hose turned on slow and put it in the radiator fill and run the engine for a good 3o minutes. Make sure the water hose is adjusted to keep up with the water coming out of the hose so the system stays full.
If it doesn't get it all out the first trip. Repeat the whole process again until it's clean.

