at idle or cruse or both??? more info PLEASE
Sounds like a story I have heard over and over when diagnosing and head or intake gasket issue most of the time a head gasket will cause overheating and/or exhaust in coolant, coolant is exhaust and/or oil, or oil in coolant. white smoke, temp guage may at time seem to go up and down or up then go down when accelerating depending on the leak and where it is, I am not saying 100% this is you issue but giving information to help you find out.
had cooling problem with my wrx years ago and all it was that Subaru service centre put the thermostat in the wrong way as they are off set and it was fouling on the inside of the block
take it for a run then check how hot your tap and bottom radiator hose are. If one is cold then it is thermostat. Subaru service centre told me my heads were stuffed till I showed them the bottom hose was hot and the top hose was cold
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