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Thread: 2001 325i Overheating but NO codes are thrown

  1. #1
    Join Date
    Apr 2019
    Location
    Comfort, Texas
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    4
    My Cars
    2001 325i

    2001 325i Overheating but NO codes are thrown

    I recently replaced the alternator on my 2001 325i and unfortunately, 'snagged' the temp sensor lead while re-installing the fan shroud.
    A few weeks later, the car overheated on my wife and this is what I found (I assume the lead finally broke while my wife was driving the car):
    [IMG]C:\tmp\sensor_lead.png[/IMG]

    I replaced the sensor and the car continues to overheat, so this past weekend I replaced the water pump.... and the car continues to overheat!!
    Overheating occurs at idle and while driving.
    The odd thing about all this is that NO codes are being thrown.

    So here's my question(s):
    Is it possible that placing the sensor lead under such a great strain could have broken one of the conductors?
    How can I test that, what should the voltage read at the sensor?

    What else could cause overheating without throwing a code?
    Last edited by jcargile; 04-23-2019 at 01:38 PM.

  2. #2
    Join Date
    Mar 2016
    Location
    Portland, OR
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    712
    My Cars
    2002 325Ci Cabriolet

  3. #3
    Join Date
    Jan 2015
    Location
    Timaru, New Zealand
    Posts
    1,132
    My Cars
    Two Z3 Roadsters
    There should be 12 V DC across the two connectors with the sensor disconnected.

    As for the overheating, did you refill the radiator and clear the air bubbles correctly? Are you getting heat from the heater at idle or only when you rev the car? Are both the top and bottom radiator hose hot/warm when the car is nearing over heat?
    1999 2.8L Z3 Roadster,
    2000 3.0L Z3 Roadster,

    There is only one thing more pleasurable than working on a Z3, that's driving it top down on a fine day.

  4. #4
    Join Date
    Apr 2019
    Location
    Comfort, Texas
    Posts
    4
    My Cars
    2001 325i
    Thanks Muzz258.... I actually thought about that this morning.
    As soon as all the thunderstorms clear, I am going to perform the air bleed and also check the voltage.

  5. #5
    Join Date
    Apr 2019
    Location
    Comfort, Texas
    Posts
    4
    My Cars
    2001 325i
    Bleeding the cooling system did not fix the issue, although it took nearly a gallon of coolant until no bubbles came out of the bleed screw hole.
    Checked the voltage going to the lower radiator hose temperature sensor and am only getting 5.03Vdc.
    Sensor_Voltage.jpg

    What in the world could be causing that?

  6. #6
    Join Date
    Jan 2017
    Location
    USA
    Posts
    284
    My Cars
    E21, E46
    How do you know the car is overheating?

    Use a thermal gun to get the real temperature.

  7. #7
    Join Date
    Jun 2012
    Location
    New Mexico
    Posts
    915
    My Cars
    2003 328i
    Too many of these threads lately. Overheating is the number one killer of these engines and you can not mess around with it.

    If the thermostat, water pump, expansion tank and cap, radiator and hoses have never been replaced, they all need to be replaced. This is preventative maintenance. It only takes one overheat to cook a motor. These engines are bulletproof other than that.

    Multiple overheats with loss of coolant is a very bad sign. Before doing any more repairs I would have the cooling system tested for combustion gases. If it passes, replace everything that you have not yet replaced. Properly bleed the system and see how it does.

    If there is anything interfering with the gauge reading such as a broken wire the gauge will read incorrectly all the time, when the engine is cold as well as hot. If it is correctly reading a cold engine it is correctly reading an overheating engine.

    The sensor in the lower hose tells the DME when to run the auxiliary fan to push some air through the rad and cool the outlet temps down. This alone can cause overheating if the car runs long enough while stationary as the radiator doesn't work well with no airflow. The DME will set a fault if the sensor is reading out of range so I would expect a DTC for an open or short here. Since you are overheating while driving I think you can rule out this sensor and the aux fan as well.

  8. #8
    Join Date
    Apr 2019
    Location
    Comfort, Texas
    Posts
    4
    My Cars
    2001 325i
    OK... here's another update (and one that should satisfy the question: "How do you know it's overheating")

    Started the car with the A/C ON and the aux came on.
    Let the car get to "operating temperature" (As alerted by Torque Pro with "Engine operating temperature has been reached")
    Operating temp.png

    Turned the A/C OFF and the aux fan turned off.

    After 20 minutes, the car began to overheat to the point where I shut off the motor. (Temp gauge was starting to climb outside of its 'normal' range)
    OH-1.png OH-2.png

    Before turning the car off, I assumed the aux fan would at least come on by itself at low speed, which it did not.
    The ONLY time the aux fan comes on is with the A/C ON. With the A/C OFF, the aux fan never comes on.
    Last edited by jcargile; 04-28-2019 at 03:33 PM.

  9. #9
    Join Date
    Jun 2012
    Location
    New Mexico
    Posts
    915
    My Cars
    2003 328i
    The fan won't come on unless the lower hose sensor is detecting high radiator outlet temperature. There is a good chance the thermostat is stuck closed and there is no circulation through the radiator.

    By now, after multiple overheats, that motor is probably hurting.

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