Car was a no heat job checked the coolant it was rusty. Flushed the system several times. And back flushed the heater core. Changed the thermostat and checked for proper function before installing. The car still will not come up to operating temp. The only way I can achieve this is high idle in a garage. is this common for this car or is there something I am missing?

 

Make sure your coolant mixture is right. If you changed your thermostat and flushed it out good, there shouldn't be an issue; except maybe you heater core actuator isn't working.