Open both the radiator and the engine block drains, and flush the system with clean water before adding new coolant. If the drained liquid is rusty, it would be a good idea to clean the system with some type of flushing solution. I use two pretty commonly available chemicals instead of a commercial flushing product. Muriatic acid (technical grade hydrochloric acid) which is used to clean up after masonry work, mixed in a ratio of 4 ounces to a gallon of water (15 ML/Liter) will dissolve rust and other metallic debris. Run the engine up to operating temperature with the acid solution, then neutralize the acid with a flush made of a strong solution of sodium bicarbonate and water. Flush with clear water, then add a 50% solution of antifreeze and water. A 50/50 solution protects against freezing down to-37 degrees F. and also slightly raises the boiling point of the coolant, even without pressurizing the radiator.
Jerry
