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Thread: KA sport overheating! How to test heater control valve?

  1. #1
    Join Date
    Feb 2010
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    Question KA sport overheating! How to test heater control valve?

    Hi all, first post..

    After reading some of the posts about KA overheating probs it seems that a lot of the times its down to the "heater control valve" .

    Is there anyway I can test to confirm before I replace it?
    Bypass? Block off? etc..

    Symptoms:

    Heater works ok.. Pipes hot..
    Overheats but only after several miles..
    Rad fan kicks in BUT rad doesnt feel hot ! just warm..
    Expansion tank cold/warm..
    No pressure in system..
    No water loss except if you ignore temp then just boils over..

    There is no pressure in the system even when the overheat light comes on.. I dont understand how there is no pressure even when normal temp could it be expansion tank cap?

    Pressure tested and all ok..

    My thinking was initially thermostat but can see a lot of heater control valve related overheats..

    Opinions and test suggestions very welcome.. TIA Kev..
    Last edited by KevinM; 24-02-10 at 11:57. Reason: clarify prob..

  2. #2
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    I do not understand how a heater control valve can cause engine overheating. All that valve does is allow hot water to flow through the heater matrix to heat up the cabin.
    To assist diagnosis of overheating, start the car from cold, and follow the heat path as the water circulates. (Or doesn't, as the case may be.)

  3. #3
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    Apr 2009
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    From your description, expansion tank cap seal could have failed, thermostat failed, or the water pump has failed. As Rolebama suggested feel for hoses warming up from cold, to work out if water is circulating properly. Or replace cap and thermostat.

  4. #4
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    Quote Originally Posted by Rolebama View Post
    I do not understand how a heater control valve can cause engine overheating. All that valve does is allow hot water to flow through the heater matrix to heat up the cabin.
    To assist diagnosis of overheating, start the car from cold, and follow the heat path as the water circulates. (Or doesn't, as the case may be.)
    I dont understand either but apparently its a common KA fault.

    http://www.rac.co.uk/forum/showpost....70&postcount=7
    I`ll find others but they are on other sites..
    Dont know posting rules.

    Out of curiosity I thought I`d try something .
    Turned the heater to cold setting and have just driven it 26 miles! with the heater in the cold setting and there were no probs and the radiator was hot this time (I mean as normal not overheat) showing to me that the thermo had opened with the heater on cold??

    Yesterday I could only drive it about 3 miles before the overheat light would show and the radiator was warmish..

    I just tried the exp`n cap and I could blow through it! So I guess thats why there was no pressure in the system.

    My theory:
    From what I understand I must have pressure in the system to allow the water temp to rise higher and that no "hotspots" in the engine could produce localised boiling (cylinder head etc..).
    So no pressure could produce pockets of steam and then of course this would pervent cooling.

    I`m letting it cool down at the mo` and then will double check I aint got an air block.

    Thanks for the replies and greatly appreciate anymore input..
    Last edited by KevinM; 24-02-10 at 17:36.

  5. #5
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    The point of the pressure cap is to raise the boiling point of the water, and only really comes into use when the coolant is at 100deg C. You should not have any localized hotspots, but if you have, I would suspect that water is not circulating properly. This could be an indication of blocked waterways, a failing water pump, or a loose drive belt. Blocked waterways can be caused by the use of some sealants. Being able to blow through the pressure cap is a definite sign that it has failed, which could lead to coolant loss, causing overheating. I don't see any reason why the engine temperature should be affected in the way you say, as, if anything, I would expect it to be the reverse of what you found. IE, the loss of the heater matrix with the valve closed would exacerbate the overheating problem as the matrix would normally help to cool the coolant. I would be inclined to bleed the system again, as it is possible you have an airlock. As for comment about radiator fan, it is not just a matter of it coming on at the right time, it is also a matter of it being able to do it's job properly, reducing coolant temp, and switching off.

  6. #6
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    Feb 2010
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    Confuses me to as to how the heater valve affects the thermo..

    My theory IF it is the HV:

    The return from the heater sits very close to the thermo about 1" .
    With heater off both pipes in/out are hot and thermo opens. (HV diverts flow and doesnt shut it off)
    Heater on return is cooler and this cooler flow close to the thermo could be preventing it opening?

    First thing I will do is obviously fix the cause of loss of pressure .
    As you say definately pressure cap.
    Make sure no airlocks.
    Recheck for leaks..

    Thanks both for suggestions they all make common sense..
    Last edited by KevinM; 25-02-10 at 11:13. Reason: clarity

  7. #7
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    I think, if you trace the heater pipes the heater is connected across the thermostat, it is on most modern cars, this ensures your heater is available before heat is being dissipated in the cars radiator. Therefore cold water from the heater returns to the main flow after the thermostat.

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