2002 Chevy Venture, 149k miles. why does it blow cold are out of vents when car is idling. even if engine temp is up to nornal temp. if I press on gas and bring RPM.s up then hot air comes out. but soon as stop at light or when I start it in morning to warm up engine gets hot but does not blow out hot air. I was told that my head gasket was leaking slightly to outside but no sign of coolant in oil yet. could that be it. I do not notice much anti freeze leaking out on to floor. What could it be. One person said maybe heater coil has air pocket. and needs to be burpped. I took off radiator cap and notice cap has oily stuff on it. Any suggestions
The vacuum-controlled air temperature flap in the HVAC system is moving when it shouldn’t. There is high engine vacuum at idle and lower vacuum as you open the throttle. I think you have a vacuum leak somewhere.
It sounds like this is the LEAST of your problems.
MC is right. I suspect the vacuum issue as well, but the air block is a possibility, if less likely. However before bothering with the heat, you need to get the head gasket issue addressed and the apparent coolant leak.