overheating

If would be good to run it for a day with pure water and 1 lb citric acid (ebay). That is what Prestone Super-flush was back when it worked well. Flush many times after, and don't let it freeze. Pull the block drains while flushing, and you may need to poke a screwdriver in the holes to free up rust. I have converted (converting) all my vehicles to Evans Waterless coolant since I abhor corrosion.