Heater blows cold air at idle but works great above 25 MPH

Hello car experts and enthusiasts, I've done some research via online forums, but thought I'd take my issue straight to my favorite hangout (ATS).

Vehicle: 2005 Honda Civic LX (Automatic)

Issue: Heater blows cold/tepid air at idle and below about 25 MPH, otherwise works great. Sometimes when I am below 25 MPH it will suddenly begin to work- I've noticed this just after going over a speed-bump. After driving for some time (30 minutes or more) the heating system USUALLY works as it should, but will still randomly exhibit the problem at stop-lights, or at idle after briefly shutting engine off.

Other notes: Had some minor work done a few weeks ago to replace a leaking rear cam seal, never noticed issue with heater until AFTER this service was done. The repair shop also "topped off" coolant.

My suspicions: Vacuum leak or air in cooling system-- or, maybe airflow valve (in firewall) sticking???

My request: Any advice for what I can do to further pin-point the problem, ways to resolve the issue myself, or what I should expect to pay to get it fixed. I don't mind doing some basic repair/replacement work, but if it requires specialized equipment or tools I will have to rely on a mechanic/shop.

Thank you!

? A vacuum switch ?

Check electrical connections to the fan heater.

Could be as simple as a loose wire.

a reply to: seattlerat

nothing happened before the repairs?

youre not going to get air in your system to have to 'bleed' out of all they did was take the radiator cap/overflow bottle cap off.

no thermostat replaced? any drops of coolant in the car like down by your feet under the dash?

a reply to: seattlerat

I would replace the thermostat...probably the easiest and cheapest fix to try.
It could be as simple as that.
Usually around $10 for that and a gasket depending on which kind you have.

a reply to: seattlerat

Does the air get warm if the car is in park and the engine's rpm is up to 2500?

Check the radiator water level.

What does you temp gauge read?

Sounds like a thermostat.

a reply to: RazorV66

Yeah. Or take the current one and put it in boiling water to make sure it's functioning properly. They're cheap, but why replace if it's not necessary?

I always boil new ones too, after buying a couple that were bunk.

originally posted by: denybedoomed
a reply to: RazorV66

Yeah. Or take the current one and put it in boiling water to make sure it's functioning properly. They're cheap, but why replace if it's not necessary?

I always boil new ones too, after buying a couple that were bunk.

Yeah that should determine if it is bad for sure.
I just checked on Autozone they are under $20

originally posted by: seattlerat
Hello car experts and enthusiasts, I've done some research via online forums, but thought I'd take my issue straight to my favorite hangout (ATS).

Vehicle: 2005 Honda Civic LX (Automatic)

Issue: Heater blows cold/tepid air at idle and below about 25 MPH, otherwise works great. Sometimes when I am below 25 MPH it will suddenly begin to work- I've noticed this just after going over a speed-bump. After driving for some time (30 minutes or more) the heating system USUALLY works as it should, but will still randomly exhibit the problem at stop-lights, or at idle after briefly shutting engine off.

Other notes: Had some minor work done a few weeks ago to replace a leaking rear cam seal, never noticed issue with heater until AFTER this service was done. The repair shop also "topped off" coolant.

My suspicions: Vacuum leak or air in cooling system-- or, maybe airflow valve (in firewall) sticking???

My request: Any advice for what I can do to further pin-point the problem, ways to resolve the issue myself, or what I should expect to pay to get it fixed. I don't mind doing some basic repair/replacement work, but if it requires specialized equipment or tools I will have to rely on a mechanic/shop.

Thank you!

If it's an air issue, one thing you can do is pressurize the cooling system with air. With the engine off, you should be able to hear then the leak and locate it. Be it a valve or a hose....whatever.

Good advice so far and I will only add that a loose belt to the water pump or a weak water pump ...best of luck

My money is on air in the system. When cool, fill the radiator to the top, then do the same with the coolant resevoir.
Let the engine heat up and cool down twice. Make sure the breather/overflow on the res tank is clear by blowing through it. Have the heat on while you fo the steps.
Should fix it. If not try a full radiator chemical flush with the heater on, just don't forget to flush the flush, or it will eat stuff up.
Some cars, like the 99 Jeep gc require you to lift the front end in order to get the remaining air out of the heater core.
It won't fully bleed off otherwise. Annoying problem to nail down.
Parking up a hill works if you don't have jack stands.

a reply to: Mandroid7

yes
have the heat on and keep the coolant topped off. give it some revs and hold it and then cap it. it creates a bit of a vacuum and you can truly top off that way. you can do it with your hand at the throttle body or have someone press the pedal. not required but it is the proper way to do it

originally posted by: seasonal
a reply to: seattlerat

Does the air get warm if the car is in park and the engine's rpm is up to 2500?

Check the radiator water level.

What does you temp gauge read?

Sounds like a thermostat.

I agree

Sound like water levels,,add water,,simple,fix

Air pocket in the cooling system,any mechanic could fix in 15min.

originally posted by: WUNK22
Air pocket in the cooling system,any mechanic could fix in 15min.

This^

My car had the exact same problem. It's got a small crack in block and it builds up a air bubble in heater core.

originally posted by: bananashooter

originally posted by: WUNK22
Air pocket in the cooling system,any mechanic could fix in 15min.

This^

My car had the exact same problem. It's got a small crack in block and it builds up a air bubble in heater core.

My car did, that is

Thanks to all of you for the replies so far- I have some things to try and will get back with what (hopefully) worked!

To answer some of the questions:

* No water on the floor near heater core location
* Thermostat was NOT replaced and engine temperature is normal (gauge shows exactly the same as it always has just below halfway)
* I did try to bring rpm up while in park and it didn't seem to make a difference, I'll try this again today

I hope it doesn't have anything to do with water-pump. I've changed a thermostat before so I could try that if adding coolant and parking on hill technique doesn't solve the problem.

You guys are great!

A vehicle as young as that could well have the more sophisticated air delivery system as fitted to very modern cars, with a series of flaps all through. They mostly go to a default setting when you stop the car...you can hear whizzes and buzzing as the system shuts down, so it could be an errant flap that maybe has grit or something similar stopping it's proper function. I take it you mostly have the setting with outside air available to mix, rather than set on recirculating air which can fog up the car inside very quickly...so dangerous.

Try the fan on full blast and mess with all the settings to make sure all the delivery systems are activated at some time including the AC and auto modes.

Just to add, the Civic Air Mode Control Motor, under the dash probably, that controls the initial function of air from the fan, that itself could give the same affect as you have if it is not sequencing properly. Replacing that motor should not be a big deal, nor too expensive.

See if there is trash blocking the heater core. remove both hoses at the engine and backflush the heater core.

originally posted by: RazorV66
a reply to: seattlerat

I would replace the thermostat...probably the easiest and cheapest fix to try.
It could be as simple as that.
Usually around $10 for that and a gasket depending on which kind you have.

Thats probably it. Thermostat stuck in the open position
Happens every time...