Q: New thermostat and truck is still blowing only warm air. It blows cold air when idling

Trapped air in the cooling system can sometimes account for this but you are indicating that you are getting heat, at times, which would indicate that coolant is flowing. There is a heater control valve and you want to make sure that the valve is fully open. If it is not fully open, delivery of heated engine coolant to the heater core will be much reduced. Once you have confirmed that the heater control valve is fully open, check the temperature of the heater hoses leading to the valve. The hoses should be hot, just like the radiator hoses. If the valve is open, the hoses are hot and the heat is still inadequate in the passenger compartment, that means the blend door in the dash is stuck in the wrong position, not allowing air flow over the heater core, and/or the heater core itself is plugged and thus not allowing adequate circulation of hot engine coolant. If you desire that a certified Mechanic from YourMechanic diagnose and fix your no heat condition please contact YourMechanic to set up a mobile visit.