My sons Honda keeps overheating and I don't know why. his transmission is blowing oil out the vent and the transmission shop says the oil is getting too hot and causing the tranny to pressure up. We have a mechanic pressure up the system twice and it holds pressure. We also replaced the radiator cap (just in case that was the problem). We can't see any water leaks but he has to keep adding water but I suppose that might be because of evaporation when it gets hot. There is no white smoke exiting the tail pipe. It has coolant in it. I just can't figure out what is causing it to overheat. What else should we check as a probable cause?