Heat is cool when engine is idle
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  1. #1
    Newbie jgray152 has not yet proved knowledgeable
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    Default Heat is cool when engine is idle

    Both my 2005 elantra and my GFs 2005 elantra both have this issue. During idle when the car is fully warmed up, even when you stop at a stop light, the heat will cool down to where it will sometimes blow cool or cold air. Yesterday while I was driving with the engine warmed up, there was not warm air blowing at all only cold air. After messing with the controls I started getting warm air again but if I stop and the engine idles the warm air begins to cool down instantly.

    I noticed the heater core hose IN was hot is normal and the output was cold. I don't believe the output should be cold correct?

    Any advice?

    ---------- Post added 11-28-2010 at 05:53 PM ----------

    Also I have been here many time before, Might know the name RotaMan. I am just having an issue logging into my original account.

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  3. #2

    Default Re: Heat is cool when engine is idle

    i have the same problem, still trying to figure out why.

  4. #3
    2003 Elantra Owner smileymattj knows about Elantras smileymattj's Avatar
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    Default Re: Heat is cool when engine is idle

    Did you make sure your coolant is filled to the full level?
    Have you replaced your thermostat with one that opens at a lower temperature? Sometimes changing the thermostat temp rating from the factory will cause problems.

    If the car is operating at normal temperature I'd say the thermostat, radiator, and water pump is working correctly. I wouldn't rule out the water pump or thermostat completely, it might be working but not flowing enough coolant to make the heater core hot. You guys being up north overheating would be less noticeable.

    There are 2 hoses that enter through the firewall behind the intake in the center. They go inside the car to the heater core. Check to make sure these hoses are getting warm.

    The water pump is on the passenger side of the car, but the thermostat and thermostat housing that distributes the coolant is on the driver side.

    The thermostat housing might not be allowing coolant to flow into the heater core, or it could be restricting the flow, due to some blockage.

    The hoses back to the firewall could be clogged.
    The heater core inside the car can be clogged.

    There is a flapper inside the the heater box that allows air to flow through the heater core. This flapper might not be operating correctly, or the temperature knob might not be working correctly to adjust the flapper correctly.

    It appears that the heater core doesn't really get direct flow of coolant. It looks like the coolant flow would prefer to go to the radiator and just gets forced through the heater core if the thermostat is closed; but if the thermostat is open, it might not get much flow. Never opened the thermostat housing so I can't say for sure.

    -My guess would be some flow restriction in the thermostat housing or the hoses in-between the firewall and thermostat housing. Or the thermostat staying open too much. But I think the place to start troubleshooting is in the thermostat housing. Make sure the thermostat closes properly and that the housing is not blocked.


    See the attached PDFs for diagrams.
    Attached Files
    Last edited by smileymattj; 11-28-2010 at 09:10 PM.

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    I am not responsible if your car blows up, use common sense and proper safety precautions.

  5. #4

    Default Re: Heat is cool when engine is idle

    but the problem is only at idle. any rpm above idle, i get plenty of HOT heat. thats the part that confuses the hell out of me...

    it has to be something on the motor, like u said, thermostat, housing, or pump. because idle = cold, any rpm over idle = hot.

  6. #5
    Newbie jgray152 has not yet proved knowledgeable
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    Default Re: Heat is cool when engine is idle

    Thanks for the documents. Anytime the engine is running, if its at idle it will be cool and if I rev it up even another 500-1000 rpm it will begin to warm up, then let it idle again and it will cool down.

    The hoses leading to the firewall, one is really warm the other is cool. I thought of the coolant and saw some coolant in the overflow, not to where it full but my guess is the rad is plenty full when hot because there is pressure in the hose but I will crack open the rad this morning.

    Is there a valve or thermostat in the heater core possibly? I was leading toward the water pump which I am not ruling that out just yet.

    I have not replaced the thermostat at all on the engine.

    Thanks guys.

  7. #6
    EXD Pro Member casale knows about Elantras casale's Avatar
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    Default Re: Heat is cool when engine is idle

    Quote Originally Posted by smileymattj View Post
    Did you make sure your coolant is filled to the full level?
    That's step one. I also had no heat at idle and found that my coolant overflow tank was low. Filling it back to the "cold" line fixed the issue for me.

  8. #7
    Newbie jgray152 has not yet proved knowledgeable
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    Default Re: Heat is cool when engine is idle

    Welp I jumped the gun on this one. Skipped the most valuable step. I check the rad and it was low. Seems to be fine now.

  9. #8

    Default Re: Heat is cool when engine is idle

    checked radiator, and overflow tank, everything is tip top shape. so low coolant is NOT the problem im having.

    gonna b taking the heater core out this weekend to flush it out make sure nothings blocking it, and probably replacing the heater hoses as well.

    im also gonna go ahead n buy a new water pump n replace it sometime soon too. esp since im still on my original, n its almost got 100k on it.

  10. #9
    2003 Elantra Owner smileymattj knows about Elantras smileymattj's Avatar
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    Default Re: Heat is cool when engine is idle

    Pete, if flow is restricted then it will be hotter if the engine speed is increased. Water pump spins as fast as the engine spins. So if there is a clog somewhere it will cause low heat at idle, then when the flow is increased it will pickup in temperature. Especially with the manual climate controls. They don't "maintain" a specific temperature. Manual is very dependent on the condition and ability of the heating and cooling system. Automatic controls would say nope it's not the desired temperature, continue working until desired temperature is reached.

    Even though it's a good place to start troubleshooting, there are much easier things to check first. I would check the thermostat housing and hoses first. If it's time for maintenance replacement, then it wouldn't be a bad idea to change everything you're planning on replacing. By the time you dis-assemble it to check it, might as well replace it.

    Still could be a problem on the air side of the heating system. As air passes the heater core the heat from the coolant is exchanged to the air via the heater core. If cold air is mixed in that hot air, it could result in the symptoms that you are getting. Then as the heat increases the cold air is less noticeable.

    Another thing that might pose a problem is not the correct coolant/water ratio. Coolant's main purpose is to prevent the water from freezing. The water will carry the heat much better. It's best to only use the amount of coolant required to keep the water from freezing. That's why in hotter climates less coolant is recommended. Owners manual and coolant bottle should have the ratios for each climate. Mine is almost fully water, it rarely ever reaches freezing here. Most times when freezing point is reached my car is in the garage where it wouldn't even get to the freezing point.

    If you are on a budget. Check in this order:
    Check heater hoses.
    Apply a water hose to one side of the heater core entering the firewall, see if it flows smoothly out. Reverse to the other pipe, make sure it flow smoothly both ways.
    Check the thermostat housing (will need to at least replace the seal once opening)
    Check the thermostat for proper opening/closing tempsthermostatTest.pdf
    Check the water pump, make sure the bearing isn't worn out (will need to at least replace the seal once opening)
    Preform a coolant flush
    Check to make sure the controls are operating correctly (time consuming)
    Check to make sure cold air isn't entering the air ducts (might require taking apart the dash)
    Last edited by smileymattj; 11-29-2010 at 10:13 PM.

    smileymattj's Garage
    I am not responsible if your car blows up, use common sense and proper safety precautions.

  11. #10

    Default Re: Heat is cool when engine is idle

    no. u dont. even tho its not that hard to take the dash out anyways.

    theres 2 small plastic covers on both sides of it. kick panels, if you will. the passenger side one reveals the inlet n outlet side of the core, while the other side has all the electronics on it.

  12. #11
    2003 Elantra Owner smileymattj knows about Elantras smileymattj's Avatar
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    Default Re: Heat is cool when engine is idle

    Cool, that makes it easy then. Sorry I was figuring it was a hassle to get to.

    smileymattj's Garage
    I am not responsible if your car blows up, use common sense and proper safety precautions.

  13. #12

    Default Re: Heat is cool when engine is idle

    it was a hassle figuring out how to get the dash of the first time... but once u figure out where the bolts r, n break the retarded clips in the back... its pretty easy. lol


    ill update this thread, and mine, as soon as i have a chance to do this, later this week.

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