My car has a pinhole leak in the upper radiator hose that i just noticed. So as a patch to get me to the parts store i wrapped some duct tape around it and filled the rad up a little. I let it run for a while waiting for the temp to get high enough to open the thermostat and see how badly it still leaked. I left the rad cap off to burp the system if any air had gotten in while the coolant level was low and let it run for about 10 min. well it didnt get really hot so i gave it a little throttle and then looked at the rad opening and the coolant had foamed up and was trickling out the top of the rad. The patch worked and the hose didnt leak but the thermostat did open because the hose was full and hot and when it started to flow into the rad it started the foaming. My question is, what is causing the coolant to foam up? Any ideas?