1999 cavalier. Daughter wrecked it, replaced radiator with salvage one. New water pump and thermostat. Engine heats, looses all coolant fluid (tank empty) but no leaks can be found. No anifreeze in oil, no moisture coming from tailpipe, and floorboard is not wet. HELP any suggestions? Fluid level stays fine while idiling, but when she gets out on the road it goes empty and light comes on.