Start by checking the coolant level. Make sure it is at almost "FULL" level. Also condition/quality of the coolant. It should be at 50/50 mix coolant/water, if you have too much water, it will not give your the heat that you want.
Check your thermostat to make sure that it is not stuck open.
Back to the quality of your coolant. If it has not been serviced properly and/or at the specified interval, sludge may build up in the heater lines causing restriction into the heater core. So at idle (low pressure), the sludge may prevent the warm coolant from entering the heater core.
Monday, January 26th, 2009 AT 1:00 AM