Find the small hoses, (approx 3/4 inch) running from the engine to the firewall. Detach both of these hoses from the engine. Using a garden hose run water in first one hose and then the other hose. It may not work, but it's worth a try.

There are 2 ways the 1st is the least effective and most expensive, have your entire coolant system flushed, this will sometimes do it and sometimes not but it is for sure going to cost some money. 2nd which 9 out 10 times will work but you have to completely remove the core and then back flush it with hot water and some type of cleaning solvent, this is give use the best results.