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what thermostat???

Discussion in 'Z3 Roadster & Coupé' started by David Cullen, Aug 31, 2014.

  1. David Cullen

    David Cullen Zorg Legend
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    have a problem guys, thermostat went last night, stuck closed, I caught it in time due to a pipe blowing off with the back pressure so was lucky in one sense, for the moment I have taken it out all together and all is fine apart from the temperature obviously. my question is, what thermostat should I use? is there a reliable one out there?
    I only changed the thermostat about three months ago as a preventative measure, so that was a great idea.... not..... anyway, any help appreciated guys, thanks :)
     
  2. vintage42

    vintage42 Dedicated Member
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    Was the stuck thermostat a BMW part or aftermarket?
     
  3. Cooper

    Cooper Zorg Guru (IV)
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    I hate stories like this, makes me cautious about changing anything! :nailbiting:
    Please let us know what brand the replacement was!
    Sorry I can't assist with the best brand or option but I hope someone assists real soon. I will be watching... @FluxMorz might be interested in this post as well, he needs a new one and may have found an option already.
     
  4. David Cullen

    David Cullen Zorg Legend
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    thanks guys, it was aftermarket part, cant remember the name but have the receipt there so will check it tomorrow.
    was thinking though.... I know the cooling fan is running all the time but I presume it kicks into a higher gear when the engine gets too hot, would it be the case that it didn't do this and that's what made the car overheat? is there a fan switch on the car that could be gone? just trying to cover all possibilities :) im not sure how the BMW fan system works to be honest
     
  5. vintage42

    vintage42 Dedicated Member
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    My 1.9 has only an electric fan on the radiator, and it does not run all the time.

    Your engine overheated because of a stuck thermostat, you said.

    From other posts here and there, the Z3 cooling system works like this:
    The Z3 uses a two way thermstat to blend the cool an warm waters in order to supply a constant inlet temperature. The thermostat does not control the engine temp, it controls the coolant temp entering the engine. A 88C thermostat doesn't mean the engine should run at 88C. Rather the inflow coolant is held at 88C, and the outlet temp will be that plus whatever additional heat is picked up on the way through.
    The fan does not cool the engine. The fan cools the radiator. The radiator cools the coolant. The coolant cools the engine. Each junction has a thermostatic control of some sort. The fan does not come on because the engine is too hot. The fan (electric or viscous) comes on because the radiator needs cooling in order to supply coolant at or below needed inlet temp. This why the fan sensor is in the radiator outlet rather than the cylinder head.
     
  6. Cooper

    Cooper Zorg Guru (IV)
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    @vintage42 I think you have it wrong. The thermostat monitors the engine temp and lets cooled water from the radiator in to keep the engine within specifications. The engine needs to run at a certain temp to reduce wear and control emissions and fuel mixture.The thermostat stays closed while the engine heats up until it reaches normal temp. If it starts to go over the specification the thermo will open and allow cooled water to get through the engine. It will do this as often as required.

    If the thermostat stays open the water is always cooled and the engine will operate below normal operating temperature. This is bad.

    If the thermostat stays closed the engine will overheat and BOOM. Not good.
     
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  7. Brian H

    Brian H Zorg Guru (V)
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    @David Cullen I have never heard of a Z3 thermostat failing shut! If it is only a few months old at least you should get a replacement/refund. I assume it was a complete thermostat and housing you replaced?

    It sounds as if you have a viscous fan, this fan will turn all the time but engages/disengages depending upon the temp of the engine. With the engine running and cool the fan will freely spin but when the engine is hot the fan will engage and run proportionally to the engine speed.
     
  8. David Cullen

    David Cullen Zorg Legend
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    funny answer Vintage, but thanks :)
    @Brian H no indeed Brian, it is more common for the thermostat to be stuck open, which wouldn't be such a problem :-/ I did buy the whole unit new and there is a viscous fan in my 1.9, so it is these two factors that I am looking at
    1. the thermostat stuck closed.... a bit strange in bmw, but not impossible, but bets the question..
    2. the viscous, how does it operate and what controls it? would a faulty switch cause what happened? and if so, where is that switch or relay?
    so that is the thought process for the moment and as usual, any help greatly appreciated ;-)
     
  9. Cooper

    Cooper Zorg Guru (IV)
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  10. vintage42

    vintage42 Dedicated Member
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    I believe the explanation of the cooling system in Post #5 as gleaned from another forum is correct.
    The thermostat is a mechanical device that responds to temperature to admit coolant to the engine. Similarly, the fan sensor is an electrical switch on the radiator that responds to temperature to turn the fan on and off.
    The thermostat operates not in response to the temperature of the engine coolant, but in response to the temperature of the radiator coolant, which the fan helps maintain.

    As summarized in the other forum:
    The fan does not cool the engine. The fan cools the radiator. The radiator cools the coolant. The coolant cools the engine. Each junction has a thermostatic control of some sort. The fan does not come on because the engine is too hot. The fan (electric or viscous) comes on because the radiator needs cooling in order to supply coolant at or below needed inlet temp. This why the fan sensor is in the radiator outlet rather than the cylinder head.

    As for the actual monitoring and reporting of coolant temperature to the ECU, that is done by a coolant sensor on the engine. But that sensor does not actually control either the temperature of the coolant in the radiator or the temperature of the engine. It reports engine temperature to the ECU for adjusting the fuel mixture.
     
    #10 vintage42, Sep 2, 2014
    Last edited: Sep 2, 2014
  11. vintage42

    vintage42 Dedicated Member
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    The viscous fan, like the electric fan, is controlled by a coolant temperature sensor, which is a switch, in the radiator outlet.
     
  12. Brian H

    Brian H Zorg Guru (V)
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    Not sure I agree with this, the viscous fan will be controlled by either a bi-metalic metal switch or a fluid which changes viscosity depending upon the temp of the fluid. There is no electrical connection to a viscous fan. Below is a video explaining the basic operation of a viscous fan, this is not necessarily the exact type that is in the Z3 but you get the idea.

     
  13. Rudyrov

    Rudyrov Zorg Legend
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    My car (98 Z3 2.8) was purchased with what I now believe was a failed thermostat, fortunatly it failed in the open position and since I use to live in Arizona I didn't have a clue. Then I moved to Seattle and quickly figured out I had no heater and the culprit was the thermostat.
    In addition to heat I gained gas mileage because the engine now ran at the ideal temperature and I assume the computer actually started to work the fuel injection system properly.

    I believe that the thermostat responds to coolant temperature on the side that the wax capsule sits. Since the capsule in my BMW sits on the engine side then it is logical to believe that the engine temperature controls the thermostat. Here is a terrific explanation:
    http://auto.howstuffworks.com/question248.htm

    I do think that the fan clutch senses the air temperature of the air coming from the back of the radiator and this causes the spring located at the middle facing the radiator to heat up and unwind. This motion causes the pin inside to move and let the fluid inside circulate to the viscous clutch. Here is a great explanation of the fan clutch that explains how that fluid actually circulates:
    http://www.howstuffinmycarworks.com/Fan_clutch.html

    Cheers and happy cooling systems!
     
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  14. oldcarman

    oldcarman Zorg Guru (V)
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    Recommend buying a manual, if you're like me that collects them to gaze at blankly, or get the cheap discs advertised on eBay. I read mine all the time, it's amazing how it helps understand other's problems. And a couple of times I've been able to assist members with the correct Bentley info from the manual. HTH JIM
     
  15. zedonist

    zedonist Guest

    Wow amazing thread, but guys simply, the thermostat stays closed until the engine reaches operating temperature aka water temperature, then water flows around the whole system and ends up in the radiator where driving air force cools the water in the radiator, if you are in traffic and there is no driving Air Force past the radiator, then the temp raises in the coolant and hence engine which changes the viscosity of the fluid in the viscous fan and allows the clutches to engage and spin the fan at engine speed to generate Air Force back through the radiator and keep the water at the correct temp, once driving again, the viscous fluid cools and releases the clutch and driving Air Force once again cools the water in the radiator, simples.............
     
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  16. Rudyrov

    Rudyrov Zorg Legend
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    I could have sworn thats what all that stuff above says?
     
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  17. zedonist

    zedonist Guest

    Yeah but disjointed and trying yo add to much science to it, I'm surprised nobody added Knock sensors, MAF and lambda just to confuse........
     
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  18. oldcarman

    oldcarman Zorg Guru (V)
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    And now we have the official Zedonist simples version for those of us that can't do simples ourselves!! Thank you oh master!! JIM
     
  19. zedonist

    zedonist Guest

    No problems Jim ;) Where's the picture of that Z3?
     
  20. zedonist

    zedonist Guest

    Plus does moving to Seattle improve cooling in a Z3, may have to try it.........
     

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