I have a 1998 Honda Civic with 155,000 that is having a heating problem. When the car is moving, the heater works fine. When I am sitting at a stop light, cool airs begins coming through the air vents. The temp gauge stays steady, but cool air comes in. I know that if your thermostat goes out you will get cool air, but generally you get cool air all the time. I am only getting cool air when idling. I do not see any white smoke coming from the tailpipe and coolant level has remained steady. Car is in good shape and is properly maintained.
Where is the temp guage steady? Assuming it is in the normal spot (about 1/2 way between H and C) then the problem is in heating system. Perhaps the heater core is getting clogged and only flows a decent amount of coolant when the motor is running at 2,000 rpm. Or a faulty heater control valve could be causing poor flow. See if a radiator shop can test the flow through your heater core.
yes, the temp gauge is steady at the normal spot. Thanks for the tip.