A few months after I bought the car(used that is, with 198,000 miles on it :eh: ) and it began to overheat. Sitting in traffic for only about 30 seconds would cause the temp to rise steadily. I discovered that the water level was going down after the thermostat was replaced, so I then replaced the radiator cap because I also discovered that water was going into the overflow tank but not coming back out once the car cooled off. and as the overflow tank filled up it just pushed the water right out the top of the tank. There are no holes in the hose that connects it to the radiator, and I'm not sure why else it would be doing that. Any sugestions?

 

Look at the surfaces where the radiator cap seals against, in the radiator neck. If there's scratches or gouges, the cap won't seal properly.