ScoobyMonty

Hi All, I really need some advice?! I have a 2006 Impreza STi. The cabin heater is blowing cold air, which is a nightmare bearing in mind that it's rained every day for the last 18,000 days... I have burped the radiator, thrice, by running the engine with the rad cap off and topping up with coolant. After I do this the heating works and blows hot air, for a short period then goes back to cold. There was a tiny amount of gunk on the rad cap and a tiny amount of oily type gunk that floated out of the rad, which could indicate a head gasket issue, however, the car is not burning oil and the oil does not have any impurities. And the head gasket was changed 4 years ago... And the coolant has been clear and free from gunk since.. Any advice?