Make sure you use a "green" coolant that not pre-mixed. The "green" type coolants have phosphorus, thats what cleans the pipes. Anything else probably won't work. I tried soaking pipes with dish washing power & hot water and got them semi-cleaned. Good enough, but could be better. (FYI- some dish washing power still has phosphates, but is being taken out). If you lve outside the US, some of those dish powders still have higher phosphates.