I have a 2006 Civic EX and the heat only blows hot when I am driving. When stopped at a light, the heat turns to cold air. When at driving speed it will blow hot. I have replaced the thermostat and made sure the coolant level is full. I still have the same problem. What should my next step be? I understand the heater core could be clogged or the heater control valve may be stuck. Any advise will be greatly appreciated.

 

This is almost always because of a bubble of air in the system. Check the coolant in the radiator, not just the plastic tank.

 

Thanks for the tip. I added coolant to the radiator and the tank and still have the issue. I will try to deal with the possibility of a bubble.

 

A bubble in the system would mean that you need to bleed the coolant system. I don't think my 97 DX has a bleeder, but my 98 camaro does. If there's no bleeder, you can do it by opening the primary radiator cap (but be careful while its hot! Don't do it with the engine running or it'll spray hot coolant all over you, and even when not running theres a strong chance it'll spray from being pressurized!)

 

This is the procedure for cars without bleeders. Park the car on level ground or facing uphill and wait until the engine is cold. Open the radiator cap and fill radiator to the top. Start the engine (cap still off) and set the heater on hot. Add more coolant to the radiator as needed. After a while as the engine warms up it will start to overflow; put the cap on at that point. Make sure there is some coolant in the plastic tank too. Test drive until fully warmed up and confirm that the heater works. Park again on level ground or facing uphill and check the radiator again after it is cold.

 

I am having the same problem with my 03 civic ex, did bleeding the system fix the problem?

 

YES... I did raise the front end and topped off the coolant in the radiator. This seems to have corrected the problem. So simple, who would have thought it. Thanks for all your help mk378.