No or little heat in a 1993 astro van?

My friend has a 1993 Chevy astro van AWD. He recently replaced the heater core and thermostat. When the van is idling, the heat is basically not there. When he gets on the gas the heat picks up a little. The temp guage shows just under a quarter most of the time. He emptied the cooling system and refilled it with 50/50 coolant and nothing has changed. I work at a garage in the area and I have no clue what else can be wrong and neither does the other people I work with. Any one have a clue?