08 Uplander Front Heat Issue

Hi there. Recently bought an 08 Chev Uplander. I just noticed that when the van is idle there is little to no heat coming out of the front vents. When I accelerate I get heat. Now, the rear heat is working fine. What's strange is that when the fan setting is set to 1 or 2 I get heat. increase it to 3 4 or 5 and I lose heat. I let the system cool and checked that rad, but when I remove the radiator cap, the fluid comes up and over (this is from cold). Does this mean air in the system? why would heat work in the back but not the front. doesn't the heat come from the same heater core? Why heat at low fan speed setting but not at high speed setting? Many questions hopefully some great answers. Thanks.

You have two heater cores, one for the front, one for the rear......

Have you been losing coolant? If yes, then an air pocket could be starting....

If no, then you could have a partially plugged heater core, restricting flow.....try reverse flushing the front heater core, and do a good air purge during refill....

Thanks for the info. I've got an appointment on Monday to have this done. I'll let you know the results.

It's been a while, but is everything OK? Did the dealer find something out of the ordinary?