I have a 1990 Acura Integra and it is overheating. I changed the thermostat but there is hot pressure coming from a hose to the heater core. I noticed that the radiator hose coming from the thermastat is cool and the upper hose coming from the radiator is hot. Plus when I turn off the car I hear a boiling hissing sound. It runs poorly on idle. There is no milky oil so what is the problem