I was walking around the neighborhood yesterday and a young man was replacing the radiator in a 1990's era Volvo. I stopped to chat and he asked if I knew why the car would not accept the water he was trying to put in after the replacement of the radiator. He demonstrated for me that he could add a gallon or so while the car was running and then it would vomit it all back up out of the overflow bottle. Any ideas?

He was using water because he was tired of wasting antifreeze and planned to add antifreeze after he solved the vomiting problem.