Heater Only Works When Driving
 

I have a 2005 civic ex that I just can't figure out. It will only blow hot air when running over 2500rpm. At idle at a stop light it will blow as cold the outside air. Engine temp get up to normal running temperature. Here is what I have done/checked so far:

-checked coolant level (it was fine) and checked for any leeks (there were none)
-Flushed out the heater core. There was no sign that it was clogged. Coolant and water came out clean as new.
-Removed thermostat to make sure it was working properly.

Same problem remains. When at idle, both hoses going to the heater core are cold so I don't think coolant is getting pumped into the heater core.

Any ideas on what the issue could be?
Thanks a ton!

Is the radiator staying full to the top? Don't just check the plastic tank, it can be full though the radiator and engine aren't.

If it's really full, the water pump rotor could be bad. That really only happens when there is severe corrosion from using plain water or very old antifreeze in the system.

I have checked the coolant level more times than I can count. When I flushed the heater core, I refilled the radiator and started the car and continued to add more coolant as needed. I did this until the the coolant level in the radiator stabilized. Is it possible that air got into the system from when I refilled it?

Bleed the cooling system with the heater valve wide open.

I have a 2005 Ex SE, mine did the same thing 2 weeks after I bought it.
Mine ended up being a blown head gasket.
I would too off the radiator, would get good heat for a day or so, then back to same issue you are describing.
No coolant in oil, no oil in coolant.
Did a block dye test ( place testing apparatus with dye in radiator neck, start car) if exhaust gases are leaking into the coolant system, the dye will change colors.

The d17a2 motor is known to have the head gasket fail right around 100k with/without adequate cooling system flushes prior.

Hope this helps

Could be air trapped in the system. Do what RonJ said, park in a driveway so the front of the car is elevated, take the rad cap off, let the car run with the heater on full blast until the rad fan cycles on and off twice.

Could also be bad water pump, but usually they will dump coolant from a bleeder hole when they fail and you would notice it under the car when you shut it off.

If its not those other things and you suspect an hg breach, dont do this, its not 100% accurate, the dye will only change if there are enough HC present to cause a reaction. Instead have a smog shop sniff your radiator, cheaper and more accurate IMO.

Other times I filled the radiator it was at a slight incline so the front of the car was lower. Turned it around and jacked it up a few more inches and I got about another quart of coolant into it. Seems to be working just fine now. Wouldn't have thought that a little bit of incline could make that much of a difference.

Thanks for all your help guys.