Moriarty -- your
antifreeze is fine. I change mine every four years with a 50/50 mixture of the Autozone Conventional Green coolant and distilled water. I add a bottle of Water Wetter as well. For the first few years my Cobra stayed in a detached unheated garage with temperatures plunging down in to the single digits. She's now in a heated garage. Below is a picture of her overflow tank that I took ten minutes ago. That's what it looks like after almost 18 years and that's what yours should look like. What you have is a small crack along the bottom weld/solder of the tank. When the weather gets really cold the crack parts and coolant dribbles out. When the car is warm, or hot, the crack usually seals up. But, it will eventually fail whether hot or cold. Now, you could try some of those magic coolant pellets that GM uses or just pull the tank and take it to a good radiator shop and they will fix it. I would opt for the latter.
