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I noticed the coolant overflow tank was overflowing when the car was hot(gausge at half way). When the car cooled down, it sucked the overflow tank dry. I just replaced the thermostat and changed the coolant. It's still doing the same thing. What could be causing this? air in the lines?

Does the pwer steering belt also run the water pump? Because i have an issue of the coolant boiling over in the overflow tank also. (sure would kill 2 birds with one stone if it does)

Replace the radiator cap. If that doesnt do it the most likely cause would be a blown head gasket (combustion gases are leaking past the bad gasket and overpressurizing the cooling system).

could also be a leak in the upper rad hose, causing your system to refill itself using the extra in the overflow tank. THis happened to me, my car would appear to overheat, the gauge would shoot up, but would be back to normal after I pulled over.

Thanks for the ideas, but i think due to the power steering belt being missing in action, the water pump is not being turned.