99 sebring's heater is not working right, get some heat when moving, goes cool when idle. I replaced thermostat and antifreeze, and flushed the heater core. Water pump was replaced last year. Could there be air in the system, if so how do you bleed it out.

ideas?

 

Driving the car should drive the air out of the system provided that your radiator cap is working properly. It may take several warm-up and cool-down cycles to accomplish that. Refill the system.

Feel the hoses leading to the heater core. If they are both warm, with one slightly less warm, that's good. If one is warm and the other not, then there's a blockage somewhere.

 

It can also be a water pump issue, flowing sufficiently only at cruise RPM.

Do you notice any temp gauge issues at all? Fluctuation, inoperable, or even mild overheating?
Have you checked all of your hoses for resiliency? Collapsed hoses do weird things sometimes, but most of the time they just burst.

It does sound like air though, especially if there are no other symptoms. A radiator cap is an inexpensive possible solution.

One other thing to check: (Heaven Forbid!) Look for air bubbles in the radiator with the cap off while idling. Head gaskets and cracks can let compressed air into the system from the cylinders.

 

Check the level at the filler neck when it's cool. It should be right under the cap. If it isn't, then you've either got a leak between the overflow tank and the filler neck allowing air back in rather than coolant, a bad cap with or without corrosion on the filler neck or leaking freeze plugs.