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Old 11-08-2010, 07:40 PM   #1
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Icon6 Heater blows hot when driving, cold when stopped? Please Help!

I'm knew to Honda-tech so I hope I'm doing this right. I own a 2003 honda civic ex. The heater blows hot air when I'm driving but when I stop it blows cold air. I'm not very car fixing savvy but I like to learn how to do these things my self. My father in law said that it is the thermostat but my car doesn't overheat, in fact the temp gauge stays right were it should, so I thought it might also be my heater core. Can someone please advise so I don't blow $500 at my dealership!
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Old 11-08-2010, 07:44 PM   #2
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

It could be your thermostat, heater core, or sadly... even your water pump.
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Old 11-08-2010, 10:56 PM   #3
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

themostat is cheap, replace that, make sure your coolant is topped up aswell. if it persists let us know, we'll try again
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Old 11-09-2010, 12:14 AM   #4
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

As previously noted, start with the thermostat. Cheap and easy to replace, it's worth a shot. Even if it doesn't fix the problem, at least it's new.

You could also try reverse flushing your heater core. It's only a matter of disconnecting a couple small hoses and flushing it out, re-connecting them and bleeding the system for air bubbles. Not the hardest thing to replace if you have to, but it's a bit of a tight spot.

If you end up having to replace the water pump, you might consider replacing the timing belt at the same time. (if it hasn't been done)


How many miles are on the car? Has it ever been flushed before?
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Old 11-09-2010, 05:19 AM   #5
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

My first thought would be checking the fluid level. Then bleeding the air out of the system. And go from there.
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Old 11-09-2010, 07:31 PM   #6
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

Thanks everyone for your helpful advice!!! My car has 145,000 miles and I have never flushed my heater core or any of my fluids except for oil. I will replace my thermostat and flush my heater core and cooling system then see if it fixes the problem. What is the best online resource to learn how fix the thermostat and flush my fluids?
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Old 11-10-2010, 01:06 PM   #7
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

Superninja: thanks for posting this. Funny enough, i was literally signing on to the site to post about the same issue myself. I'll take the same advise and post back as well to keep the forum updated. Best of luck...
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Old 11-10-2010, 08:45 PM   #8
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

Quote:
Originally Posted by superninja View Post
What is the best online resource to learn how fix the thermostat and flush my fluids?
As far as a coolant system flush, you can use this DIY from the FAQ's
Quote:
Timing Belt / Coolant Flush / Water Pump: Link
The thermostat is really easy. It can only go in one way.


Flushing the heater core is easy, too. There are two rubber hoses that are connected to it. The top one is the inlet hose, the bottom one is the outlet. Disconnect the hoses. Reverse-flush by putting the flow into the outlet of the heater core. (it's easiest with the hoses attached at the core, disconnect them at the other ends) Then put everything back together, top off the radiator and bleed the system for air bubbles.

Turn the temperature control **** all the way to hot, start the car with the radiator cap loosely on and wait for the radiator fan to cycle a couple times while keeping an eye on the coolant level. Turn the car off, top off the coolant again, put the cap on securely, start the car and check for coolant leaks.
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Old 11-10-2010, 09:50 PM   #9
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

I'm having the same exact problem with my '02 Si. My heater control valve works fine, I had the thermostat replaced, I bled the system of air, and I still don't have heat until about 4,000 RPM. It's not a blend actuator because heat will come through, and I don't have any coolant leaks anywhere. What the hell could be causing this?! Water pump sounds fine and the car doesn't go over operating temperature. It doesn't take longer than normal to warm up to operating temperature either.
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Old 11-14-2010, 03:06 PM   #10
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

I had the same issue with my 04 boosted EX with 105000miles on it. I replaced the thermostat and radiator cap, flushed the entire cooling system and cleaned out the overflow tank and hose and I have not had an issue since. Just make sure you take the time and get all the air out of the system, it took me a few times of driving the car around to get all the air out of it.
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Old 01-08-2014, 10:18 AM   #11
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

My car has the same problem: hot to cold, cold to hot in a matter of a few feet of car movement.

The question I have in regards to a clogged heater core is this: If I am having hot air (when car in motion) then there must be warm coolant going through the heater core, so it can't be clogged can it?

Even if it's partially clogged there is no way the warm coolant in the heater core can cool down that fast enough to cause the heater to blow cool air after sitting motionless for a 30 seconds like my car does.

To me, the cold air has to be caused by the heater core that no longer has warm coolant in it. Now what would cause the heater core to drain at idle but fill up when the car is in motion?
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Old 07-26-2014, 05:41 AM   #12
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Default Re: Heater blows hot when driving, cold when stopped? Please Help!

Forgot to post the resolution: Honda tech found the inlet hose going through the firewall was bent and was restricting coolant flow.
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