You need to locate the leak and repair the problem. Go to the local parts store and ask if you can rent a cooling system tester. It is simply a pump that goes on the radiator and puts pressure in the system. When the system has pressure, you should be able to find the leak. Check around the water pump to see if there is a leak there too. Remember that coolant can travel.
Let me know what you find.
Thursday, January 17th, 2013 AT 5:14 AM