1999 GC Heater Problem

Have a 1999 Grand Cherokee Laredo and when I turn the heater control knob to bi-level to get some floor heat for my feet I get nada. Heat coming from vents is alright in the center vents and weak out the sides. Any suggestions? Vaccum probelm or a heater door issue?

It could be either. What happens when you turn it to full floor or defrost? Bi-level really doesn't give much flow to the floor ducts.
I would first check the vacuum line for leaks, it's the large hose that exits the intake manifold.