Idle speed effects heater output

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.