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Thread: Sporadic Overheating Problem

  1. #1
    Join Date
    Oct 2005
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    Default Sporadic Overheating Problem

    Hello, I have a 92 735i. 2 months ago I had an overheating problem and it was the waterpump, also noticed the fan clutch was slow so I replaced both and also changed the thermostat and all hoses and belts... Put in BMW coolant and bled the system of air several times and it held temp great for 2 months... All of a sudden while sitting still the temp creaped up... didn't make it to the red... When I moved the temp dropped a bit... I checked the coolant and all was fine... no leaks in the system... Coolant bubbles out of the return hose into the resivor... Top radiator hose is hot so the thermostat is opening and coolant in going through the radiator... Usually I see the temp rise while sitting still and if I put the car in nutral and rev it up the temp drops... Fan clutch seams like it is working great and blows hard... Sometimes while driving the temp will creap up a small bit past center and then back down a bit below center and it keeps doing this... Usually at 60mph the temp stays dead center but normally before the problem the temp would be a tad less then center... Sometimes nothing happens and the temp stays perfect, even while sitting parked but other times the temp jumps up... It's strange cause all parts are new and were working fine for 2 months...

    What I was thinking is the thermostat is slow or going bad... normally I would think the fan clutch but it seams to be working fine and strong and stops quickly after shut off...

    Does the temp sensor only do my gauge and if so could this be a possible problem and in reality the car isn't overheating and only my gauge is going because of the sensor...

    Seams like I have covered all bases except checking the flow through the radiator but I do know the top hose is hot... If this is a possible problem how do I check it and is the check reliable...

    I have another thermostat so I may pop it in tomorrow... I have read about drilling a small hole in it so air can be bled easier... is it worth doing and if so where to drill it... any pics would be nice...

    Thanks everyone for your help...

  2. #2
    Join Date
    May 2004
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    Default

    Does the temp sensor only do my gauge and if so could this be a possible problem and in reality the car isn't overheating and only my gauge is going because of the sensor...
    Yes, there are separate temp sensors, the one for the MOTRONIC is a different one. Maybe you change this one or might be possible to check it.
    T-stats unfortunately are cheap stuff and often even new ones do not work properly or break, especially aftermarket cheap ones.
    You can check it by usung a water pot in the kitches and heat the water up and check with a thermometer when it opens.

    The symptoms - as you say - hint in direction of the fan clutch, but you say it works. Is the belt tensioner o.k. for that?
    Is the auxiliary fan working in 2 steps, low and high?
    Also check if the return hose from the radiator to the coolant expansion tank is clean and free, so that there is a constant return flow to the expansion tank. When you open the expansion tank cap, you should see a steady stream of return flow of coolant.
    Gale has a good description on his page with pics, link on my website below.

  3. #3
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    Oct 2005
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    Default

    Quote Originally Posted by shogun
    Yes, there are separate temp sensors, the one for the MOTRONIC is a different one. Maybe you change this one or might be possible to check it.
    T-stats unfortunately are cheap stuff and often even new ones do not work properly or break, especially aftermarket cheap ones.
    You can check it by usung a water pot in the kitches and heat the water up and check with a thermometer when it opens.

    The symptoms - as you say - hint in direction of the fan clutch, but you say it works. Is the belt tensioner o.k. for that?
    Is the auxiliary fan working in 2 steps, low and high?
    Also check if the return hose from the radiator to the coolant expansion tank is clean and free, so that there is a constant return flow to the expansion tank. When you open the expansion tank cap, you should see a steady stream of return flow of coolant.
    Gale has a good description on his page with pics, link on my website below.
    Belt is tight...

    I have never heard the auxiliary fan kick on or hear it run... My air conditioner isn't working so it doesn't have that extra draw on the system... If you have a link with pic to the switch that may be bad that would be helpful... Also shouldn't I hear it running on a hot day right after turning the car off?

    Return to the expansion tank has a steady flow but I am going to drill it out just for future problems... 1/4 drill bit correct?



    Strange the car can sit at idle and be fine and then all of a sudden the temp will creap up... reving it up lowers the temp


    When I rev it up and the temp drops that tells me there is a flow problem... either air or coolant not flowing fast enough... Could speeding up the flow of coolant by reving it up affect the thermostat... by the way it is an after market thermostat as I figured they were all the same...

  4. #4
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    First of all you have to get the aux fan running. It always runs when aircon is on and switches on at a certain temp. It has 2 speeds which is controlled by a relay. Check that
    http://www.nmia.com/%7Edgnrg/page_20.htm

    Also use the search on top with "aux fan" and you will get a lot of info.

    "Return to the expansion tank has a steady flow but I am going to drill it out just for future problems... 1/4 drill bit correct?"

    If there is a steady flow, it is o.k., be careful if the tank is old, the plastic gets very brittle and usually it cracks easily.

    "Also shouldn't I hear it running on a hot day right after turning the car off?"

    No, in our old cars off is off.

  5. #5
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    I just copied something for you which might be of interest for you too:
    http://www.bimmerboard.com/forums/posts/223546

  6. #6
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    What brand of fan clutch did you install?? Probably not your problem, but I put in a new Behr clutch to replace mine and it was a factory dud- broke right out of the box.

  7. #7
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    Shogun thanks for you help...

    The brand I bought from AutoZone was Torqflo

  8. #8
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    The Aux fan isn't coming on when the car starts... I let it sit and of course the temp wouldn't go above 12 oclock when you want it to...

    Did the jumper test and low doesn't work but high does... Fuses are fine and when I put the jumper on the low side I can hear the relay click so it looks like the ballast resistor has gone out...

    I am a bit confused with the pictures... is the guy checking the Resistance directly at the ballast or just at some plug? From the first picture it looks like the ballast resistor is in another location and I can reach it by only removing the kidney grill... I have delt with ballast before for lighting so Ive never seen one made into a plug like what he is testing it at... Taking the kidney grill off looks easy but to get to that plug looks like a pain...

    Shogun buddy you have been a great help again... I owe you!
    Last edited by Visions; 07-12-2006 at 08:58 PM.

  9. #9
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    Just thought of this... since when I jumpered the switch and High would turn on, wouldn't it turn on high when the temp went up and cool it back to normal like it should... maybe the switch is bad too and it never kicks on...
    I can see getting by with low not working but high must work

  10. #10
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    Resistance directly at the ballast or just at some plug
    On the plug you can test low and high speed, just bridge the contacts as shown. If it works this way, then probably the relay is out of order.
    It's usually the resistor killing the low speed and the fuse does not like it when the fan starts off in high and blows.


    Sometimes the magnets inside the motor case come loose & jam it from rotating, reach in & see if you can give it a spin manually. If if spins freely, then try jumpering the hi/lo switch on side of the radiator.

    http://www.bmw4life.com/auxFanR.htm

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