Everybody likes to see clean coolant in the recovery bottle and/or when pulling the radiator cap. These flushes and/or chemicals only help keep properly maintained systems clean. If the system has been neglected or abused (never changing coolant, mixing incompatible coolants, or running straight water), these processes and chemicals are not enough (will only get rid of the super fine or "silty" parts of the contamination). The big stuff stays behind and can only be cleaned after engine removal/disassembly. I recently tried flushing an engine that had the cooling system abused by the PO. It had been run with water only for several years. I used the Prestone chemical flush product and flushed it using the flushing tees & garden hose method. Water came out dark brown initially but after over an hr of flushing it turned clear. After flushing I filled with new coolant & drove it. The coolant almost immediately turned dirty again and overheating issues persisted. I removed engine, tore it down, knocked out the freeze plugs and here is what I found:







And the pile you see there is only a fraction of what was in there. I blasted this thing with a pressure washer from all different angles and from all directions. After that I reassembled the engine with many new parts (documented here) and now it runs as cool as a cucumber. I've put around 7k miles on it so far & all is well. Tim