Have coolant system checked for an air lock. There shold be air bleeds on higher parst of engine, try bleeding air while running to get air out.
January, 15, 2013 AT 2:28 PM
Had this exact issue. My shop replaced thermostat. Problem persisted and I had to continue to add coolant to reservoir over a 3 day period. But no heat at idle continued.
Brad1 (2CarPros) replied April 6, 2009 to my issue and described in detail how to remove air from cooling system.
I followed the air purge process and problem was resolved. I spent $160 needlessly on stat, labor and coolant. I suspect it was air in system all the time. One symptom was a gurgling or bubbling noise behind glove compartment at start up in addition to no heat idle.