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Honda Civic lowheat when stopped

I have a 2003 Civic, 145,000 miles, 1.7 engine. Replaced radiator, thermostat, head gasket, water pump and timing belt in last 6 months, which fixed overheating. When the car is moving I get heat, but when the car is stopped, cooler air comes out of the vents. Putting the car in neutral and getting the RPMs up to 2,000 does not improve the heat. Blower motor speed does not change. I ran flush / water through the core and the flow through the core is good, no obstructions. Any thoughts on what to check next? Thanks much. Hope everyone is having a nice Thanksgiving holiday.

Did you bleed the air out of the cooling system? The best way I found to do this is to raise up the front end on ramps or an inclined driveway to insure the radiator filler neck is the highest point in the system and run it with the radiator cap off, varying the RPMS and adding coolant.

I agree with @BustedKnuckles. I’ve used this technique for many years with great success.

In the morning when the car is cold, remove the radiator cap. If it is not full you have air in the system.

I have an '03 Civic EX and experienced the same symptoms 2 years ago. It took awhile to figure out, it was a small leak. When the motor warmed up the heat expanded the materials and a pressure check was OK. But, sitting overnight the car failed the pressure check the next morning. It wasn’t easy to find the leak but it was a bad clamp in my case. You have a leak allowing air into the system.

It could be a pin hole leak in a hose, a cracked nipple on a fitting, a small radiator leak, but somewhere there is a leak. Once I found my leak, I refilled the radiator every morning until it finally stayed full as all the air had purged out. Then I put a couple of inches of coolant in the overflow tank and everything has be stable since.

Thanks for the help. I jacked the front up and ran the engine without the radiator cap till it warmed up. I now have heat at idle.