Why is my 2003 mitsubishi eclipse is overheating? At first I thought that it might be the radiator, so I replaced it with a new one but the car stills overheat. Then somebody told me it might be the thermostat. So I changed the thermostat and it still overheating The old radiator had a crack on the top next to the radiator cap. So I replaced it with a new one hoping that it would solve my problem but the car still heats up.