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Old 12-08-2009, 01:39 PM   #1
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05 RX-8 heater blows cold

heater problem 2005 RX-8, vent on driver's side is blows cold and vent on passenger side blows luke warm. checked temp control function (utilizing test mode with radio buttons) and temp control advances between 1 and 16 (one number at a time). both the inlet and outlet tubes at the heater core are extremely hot. Any idea problem? how do I test the blend door actuator? Could it be anything else?
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Old 12-09-2009, 02:40 PM   #2
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found the actuator on the driverside. It moves when you change the temp control switchs. I plan to check the air cabin filter next. Still don't believe what my Mazda dealer said that the heater core needs flushing since both the inlet and outlet tube at the heater core are extremely hot. He wanted $515 to flush the heater core which from what I could see could be done from access to the hoses from under the hood without removing the heater core.
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Old 12-11-2009, 07:55 AM   #3
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I have the EXACT same problem, and am searching the forums to try and find a solution. If you find it, please let me know!
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Old 12-26-2009, 04:19 PM   #4
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We fixed the problem. The heater core was stopped up. The dealer recommended changing the heater core but that was NOT necessary. We took off the two heater core hoses and poured about half the radiator flush direct into one of the hoses going to the heater core. We let the flush sit in the heator core for about 30 minutes then reconnected one of the heater core hoses and poured in the remainder of the radiator flush and hooked back up the second heater core hose. We started the car and let it run with the flush in the system until the heater fan started blowing hot from all vents. It took about 45-60 minutes to unclog the heater core with flush running through the system. It seemed to help more by running the rpm up to about 2000-2500 instead of letting it run at an idle. Afterwards we removed the drain **** from the bottom of the radiater and flushed the system out twice.

It works great now!
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Old 12-27-2009, 05:50 PM   #5
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Thanks for the write-up Phil.
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Old 11-30-2010, 07:11 PM   #6
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I'm reviving this thread in the hope that it will help some poor schmuck like me :-)
My RX8 is one of the finnicky ones... engine recall? yep. clutch pedal snap? yep etc etc... so I'll just chalk this up to that as well.

Last year I was having the same problem as shadetreephil. I tried to flush out the heater core and it didn't work - still had cold on driver's side, lukewarm on passenger's side. Quotes from dealers for a new heater core ran from $1000-1800 and it wasn't clear if this was even the problem. I chattered my teeth and dealt with it last winter (in Boston - COLD).

This year, we finally hit the 30's and I was reminded of how miserable the winter in my car is. So I tried again.

I was having the hardest time understanding why it was warm on the passenger's side and cold on my side. And then I realized - the heater core still allows fluid to pass from the top tube to the bottom bypassing the rest of the core. So the heat I was getting was simply from the tubes and not the core itself.

I disconnected the tubes to the heater core (easy to access, below the windshield wiper fluid reservoir under the hood) and using garden sprinkler pieces hooked up my hose I flushed it out. There was lots of rust colored fluid out like last year and it took multiple flushes to get all the small rust pieces out. This time, I also did two treatments with LimeAway, each for about one hour each. This apparently is risky - looking on the web, there are people warning of it wearing away at the metal and leading to a catastropic leak... but most people reported that they didn't have this problem. Just make sure to flush VERY thoroughly after using to get out all of the limeaway (or CLR, etc).

The other thing I did differently this year was to flush in both directions. I also filled with coolant from the tube closest to the passenger's side, which is the top of the core (if you look under the dash you'll see the tubes cross so it is not immediately clear which is the upper and which is the lower tubes).

Then, I drained the whole coolant system. I had never done the basic flush, but it was much easier than I imagined - put the car on ramps, undo the screw which is accessible through a hole on the panel under the front of the car, replace screw, fill... that's it. I filled with distilled water, drove around, and drained it two more times before the final fill with 50:50 coolant:distilled water.

On the first drive around, I noticed that the driver's side heat was much better than before - and by the third time, I couldn't even hold my hand to the vent for long because of the strong heat. Believe me, this is a major improvement and will make this winter much nicer (I'm a reluctant Southern California to New England transplant).

If you need any more details, feel free to PM me.
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Old 12-07-2010, 02:57 PM   #7
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Good job cleaning out your heater core. Most dealership, well techs, suggest replacing parts instead of fixing, cleaning, or repairing then. Lazy asses.
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Old 12-21-2010, 10:22 AM   #8
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hi guys has anybody done a walk through for this as a novice i wouldnt even know where to even start or look , i have the same problems and mine isnt hot through feet either and the wife is complaining the car is cold . i look forward to your help guys .
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Old 12-26-2010, 11:22 AM   #9
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ohgoon2006

Check out Balthus's explanation. If you have trouble understanding what to do, feel free to call me.

Phil 817-419-9771
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Old 12-30-2010, 12:55 PM   #10
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How many miles are you guys hitting when you are having the issue with the heater core. Mom Complained her 05 shinka was not heating right the other day. She apparently drove 15 miles after warming up the car with hardly and heat. What should I look for to be sure the heater core is the issue before taking on the flush!
Thanks for the help!
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Old 12-31-2010, 05:03 PM   #11
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heater not blowing hot air

ours never heated up, no matter how many miles we drove it. Based on the number of people with this problem, I hate to tell you, but I would be willing to bet it is the heater core.

The engineering design is poor on the heater core. Both the inlet and outlet tubes are on the same side of the heater core, so the water is not forced to go through the heater core vanes. And since the vanes have much smaller passage ways, the water seeks the path of least resistance and leaves debris behind to clog the heater core vanes. This problem is also compounded by how they designed the core to sit in the housing assembly at a slant in the middle under the dash.

Better get ready to flush the system, and since the water can go from the inlet tube to the outlet tube without going through the heater core, flushing isn't as quick and effective as it could be.
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Old 03-22-2011, 06:28 PM   #12
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im having the exact same problem. i am a total noob to cars. dont even know how does heater core looks like. i brought it to the dealer and it says i need to replace it too
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Old 03-23-2011, 08:03 PM   #13
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Maybe your thermostat is not functioning properly, have it changed. It is inexpensive and does not take very long to replace. Your blower motor may have a loose connection but if it works on all other settings I would not worry too much about it now. As for the thermostat, i would give it my immediate attention. Check it out. Hope this helps.
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Old 03-23-2011, 08:36 PM   #14
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Maybe your thermostat is not functioning properly, have it changed. It is inexpensive and does not take very long to replace. Your blower motor may have a loose connection but if it works on all other settings I would not worry too much about it now. As for the thermostat, i would give it my immediate attention. Check it out. Hope this helps.
Thank you so much. I will try to do the steps by phil first by asking a friend of mine to do it for me
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Old 03-26-2011, 01:10 PM   #15
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Just wondering has anybody thrown a check engine light from this? I have a similar problem to this. I started my car this morning and the check engine light went on and I had the heat on but all it blew out was cold air on both sides. I started it again a few hours later and the light was still on and still no heat but then I kinda accidently got on it in 1st gear while I was making a turn and the light went off and my heat started working again......I'm kinda lost on what to do.
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Old 03-26-2011, 03:32 PM   #16
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Check what code it is first. It may not be from that.
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Old 03-26-2011, 08:26 PM   #17
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have you replaced your oil cap? there's a recall on 04-05's model. i had the same problem too. the CEL light was on and there's nothing wrong. took it to inspect and they said there's a evap.
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Old 03-26-2011, 09:00 PM   #18
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Well the CEL turned off by itself now so I wouldn't be able to scan it if it's not on anymore. Did some searching around the forum and realized the switch for the temperature is broken/breaking cause when I held the radio power button and the scan up button the temperature only read 0 or 16 no matter what temp I had it set on so that explains that. The CEL has to be unrelated so I guess I'll have to scan it next time it comes on hopefully it's nothing major cause the car was driving normally even with the light on.
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Old 03-27-2011, 03:18 PM   #19
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We had the problem return this winter. This time we used CLR (bathroom cleaning product designed to remove calcium) instead of radiator flush. They don't recommend using CLR on aluminum, so we poured just a small amount on the outside of the aluminum pipe to see what it would do. There wasn't a harsh chemical reaction so we went ahead and poured the CLR in into the heater core. CLR works quickly so we only let it set for a few minutes, then flushed LARGE amounts of water through to make sure all the CLR was gone. We had great results, much better and faster than using radiator flush.
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Old 05-11-2011, 08:01 AM   #20
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How did you remove the heater core to clean it?

Found ... I didn't see Balthus' post.

Last edited by Cromax; 05-11-2011 at 08:13 AM.
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Old 05-13-2011, 10:44 PM   #21
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We didn't remove the heater core. Instead we disconnected the hoses under the hood and used a garden hose (with a spray nozzle on the end) to shoot water into one of the heater hoses and let the water run out of the other hose. On our car, the hose connection closest to the passenger side leads to the top of the heater core. We poured the CLR in that hose, waited about two minutes and then flushed the CLR out with the garden hose nozzle
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Old 08-20-2012, 10:04 AM   #22
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I have the same issue...cool air blowing....had a CEL on so i disconnected the battery for 10min to clear the code. after reconnecting, i had HOT AIR! then after the next restart it is back to cold air!! can anyone shed some light on this?
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Old 08-20-2012, 11:21 AM   #23
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What code did it throw?
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Old 08-20-2012, 11:38 AM   #24
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What code did it throw?
I do not know...
I just got the car and the light was on. so I figured I would clear the codes and start from scratch. Since I cleared the codes the CEL has not come back on.
Is it just coincidence that hot air came out right after I did this or is there a "sensor" or something for the blower door? After the very next start up it was back to cold air.

Thanks for your response!
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Old 12-07-2012, 11:16 AM   #25
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Is their a video or a better explanation on how to drain the heater core? I don't even know what hoses to pull off and drain. If anyone could help it would be awesome because the temperature outside is slowly getting colder and colder and I have to heat on my drivers side and i'm freezing!! Thanks!
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Old 12-07-2012, 11:16 AM
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