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Thread: Thermostat Woes! Thoughts and Opinions Please!

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    Default Thermostat Woes! Thoughts and Opinions Please!

    Ok here goes - it's that time of year again - outside temperature is ranging between -2 on a good day and -10 on a bad one for the last two weeks and yet again my temp gauge is playing up. After reading several threads my understanding is that when the thermostat is working correctly it should with a bit of give and take read 88 degrees regardless whether I'm going uphill/downhill, urban/motorway driving, heaters hot/cold and any blower speed and furthermore whether I'm moving or standing idle. Please correct me if I'm wrong. Well anyway, last February, after noticing my temp gauge took ages, sometimes hours, to warm up and then only hitting 75 degrees I changed my thermostat. What a difference! Internal heaters remarkedly improved and the car running at the aforementioned parameters. Well this lasted for 5 months and then I noticed whenever I went downhill the temp gauge would start dropping. The further downhill I drove then the lower the temp would go. If the hill was long enough it'd even drop to zero and living in the South Wales valleys there are quite a lot of hills. Well fair play to my local Vauxhall dealer, who unlike some are out of this world, they offered to replace and fit a new thermostat as it was guaranteed for 12 months even though it was originally fitted by a mate. Bingo! Job done, everything was working great untill about a fortnight ago when I noticed that if I was standing idle ( with the heaters on no.3 setting which is my norm. and bear in mind the current outside temperatures ) the temp gauge would slowly drop to zero. I should add that I'm a taxi driver and with the current economic climate I spend a lot of hours sitting on idle. Well I automatically assume thermostat must be dodgy again. Fair play, off to the dealers, bob's your uncle, new thermostat fitted and disaster. Not only does the thermostat operate at a lower temp, say 84 degrees, as well as having slightly cooler heaters, same symptom of temp gauge dropping when idling, the car now loses temp when driving downhill. So I inform the dealer that they must have had a dodgy thermostat and also when they fitted it they hadn't put the egr cooler back properly as it was hissing exhaust gasses which could be visibly seen and heard. No problem says the dealer ( see, I told you how brilliant they are! ) we'll order a new thermostat and try again and refit the egr cooler pipe properly. That was 2 days ago. Yesterday, they fitted a new thermostat, sorted the leak and guess what - exactly the same problems. Now I know it was definitely changed because cynical ****** that I am - I marked the old one. What do I do? I think it's a dodgy batch of thermostats myself but if it's not and they've fitted two which would suggest that it might be something else then what could it be? I'm apprehensive about going back a third time but am at my wit's end. Opinions and suggestions required please. Bear in mind it's a working car and just in case you think I've forgotten to say what model it is - I haven't - it's a 55 plate 120 bhp 1.9 CDTI Vectra with a Z19DT engine. Thanks in advance.

  2. #2
    Full Member GazVXLINE170's Avatar
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    Vehicle : Insignia

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    Sorry for the daft question but is it only when you go down hill that is starts to fall? If so is it because you aren't using the throttle (for obvious reasons ). Tbh if it's only doing it in that situation then I would say it's probably normal. If you aren't pressing the throttle and you are in gear (recommended when going down hill) the fuel is cut off hence no heating of the cylinders.

    Seems normal enough to me.

    I have a faulty thermostat and when I sit in traffic it will rise to 90, when I hit the motorway it will drop. Don't forget that temperatures have been very low generally too, and deisels are sensitive to this.

    Gazza4
    Insignia VX-Line Nav CDTI 170.

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    Thanks for the reply, no it's not only when going downhill, it's also when standing still. So therefore, if I was stuck in a traffic queue unlike your gauge mine would slowly drop. Hypothetically, I could drive from Cardiff to Birmingham and if I was stuck in traffic for say half an hour then my gauge would drop to zero. Also, when my original replacement thermostat started exhibiting the symptom of dropping temp when going downhill in July the replacement cured this.

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    Full Member GazVXLINE170's Avatar
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    Vehicle : Insignia

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    Quote Originally Posted by bigade1 View Post
    Thanks for the reply, no it's not only when going downhill, it's also when standing still. So therefore, if I was stuck in a traffic queue unlike your gauge mine would slowly drop. Hypothetically, I could drive from Cardiff to Birmingham and if I was stuck in traffic for say half an hour then my gauge would drop to zero. Also, when my original replacement thermostat started exhibiting the symptom of dropping temp when going downhill in July the replacement cured this.

    Is the fan switch working? Is not stuck on?

    Gazza4
    Insignia VX-Line Nav CDTI 170.

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    Thanks, I thought about that one too but it seems ok.

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    When the temps were -5 my heater would drain the heat out of the engine faster than it generated it when at idle or slow traffic. The gauge would slowly go to zero when on position 3 as yours. At any decent speed it would go back up to 85c and stay there. It is a new thermostat.

    So long as your thermostat goes over 80c and stays there when driving over 20mph I think this is normal.

    All these thermostat changes of yours prove it IMHO.

    From the number of very recent threads on the subject you are not alone in being surprised that the engine can cool whilst running. As soon as the weather warms up the problem will go away I'm sure!

    A Toyota 2.2 TD that is designed for cold climates has an idle speed up switch which also operates a valve on the exhaust which restricts the exhaust to boost heater output and warm up. In cold climates this a particular problem at low speeds. This is a pic of the vacuum operated exhaust restricter from that car, just for interest.....
    It also has a flap that takes heater intake air from behind the radiator - that gives really toasty hot air - but you have to remember to change it as soon as the temp goes above 5c.
    Last edited by mozzer99; 9th December 2010 at 18:37.

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    Thanks for your thoughts Mozzer but shouldn't the engine stay the same temperature no matter what? Also why does it lose temperature driving downhill? When my first replacement thermostat started doing this in the summer it's replacement didn't drop temp. when driving downhill.

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    Regular Member LADmachining's Avatar
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    Quote Originally Posted by bigade1 View Post
    Thanks for your thoughts Mozzer but shouldn't the engine stay the same temperature no matter what? Also why does it lose temperature driving downhill? When my first replacement thermostat started doing this in the summer it's replacement didn't drop temp. when driving downhill.
    In theory, yes, it should stay at the same temperature. In practice, as you are seeing, this is not the case.

    As already mentioned, when you are coasting down hill in gear, the engine ECU stops injecting fuel into the engine to reduce emissions. In this instance, no heat is being generated in the cylinders, so the heat remaining in the engine and cooling system is being removed by the heater, resulting in the coolant temperature dropping. Even if the thermostat closes, the temp will still drop as no heat is being produced to replace that being taken out by the heater. The same will happen in a petrol engined-vehicle, but will take longer.

    The same happens when the engine is idling or driving slowly or at low load. In this case the process doesn't happen as fast, as the fuel burning in the engine is producing heat. The net result though is still a drop in coolant temp as you are taking out more heat than the burning fuel is putting in.

    This phenomenon is more pronounced in the winter due to lower ambient temperatures. The temperature differential between ambient and your desired cabin temperature is greater - the heat has to come from somewhere, and it is all taken from the engine in this case. ;-)

    This is one of the only disadvantages of having a more fuel-efficient Diesel engine - less heat is rejected to the cooling system due to it's increased thermal efficiency.

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    Nice one LADmachining. Thanks for the info. If that's the case then I wished I had left it alone because the last 2 thermostats are running approx. 4 degrees cooler and it has had made a noticeable difference to my heaters.

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    mine has been playing up to peeps, defo because of external temp drops same thing last year when we had that cold spell .....you could try partially blanking of the front grill like in the old dayzzzzz......make sure air intake is clear though....

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