heater not blowing hot air

my heater is not blowing hot air. it was fine this past summer but has steadily gone down hill. i changed the thermostat and radiator cap and now it is blowing warm air. i have been told it might be an air pocket.the motor is a 4.3 vortec

1. you're in the wrong forum.
2. it could also be a blockage in the heater core. If you get mineral deposits, it can easily plug up the heater core, causing the heater to function less and less until it stops altogether. There's products you can buy to dissolve buildups when you flush the cooling system. Next flush, try that.