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    #16
    There is no need for the hole in the thermostat with the M42. There is a reason that it does not have one. That reason is the little bypass groove in the thermostat housing that connects the two cavities. It is VERY common to have it so completely blocked up with corrosion that first-timers don't even know it is there.

    You could have a big air bubble in the head that is sitting at the sensor. You could also have a jammed up radiator or other passage. Was the water pump you replaced totally busted? Maybe the thermostat got installed backwards (it is pretty a pretty common mistake)?

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      #17
      I'm running the car jacked up as I'm typing this. I'm hoping that it is just a bubble is trapped in there somewhere. I know that the M42 has a notoriously tough coolant system to fully clear.

      The thermostat was installed correctly and I did test it before installing it. The water pump I replaced had a metal impellar, so I assume it was replaced during the car's lifespan, but I have no idea as to how old it was. Its a 25 year old car, so I know it is a total gamble, which is why I didn't balk at replacing the thermostat and water pump. Is there a specific way to know for sure when the full coolant system is cleaned out of bubbles?

      Thanks for all the help so far guys.

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        #18
        I've had to run the car for some time to let it warm up then drive it around then jack the front up and fill and rebleed at the top. I'm not sure what it is about the system but it sounds like you have an air bubble somewhere in there and you will just need to top of and bleed some more. Most important part is to stay patient!
        The fun never ends :-?

        89 318iS
        R.I.P 89 325i coupe
        89 335iC

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          #19
          Don't open the bleeder while bleeding. Just leave the cap off. No way to tell if there are bubble still, just have to try and see.

          Rev the engine up a few 1000 rpms and hold it for a few seconds ever few minutes or so. This will circulate coolant and move any air to the highest point (which should be the coolant cap with the car jacked up).

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            #20
            You can also try to "burp" it.
            This isnĀ“t something you would really like doing, but burping is (high) revving up the car directly after cold start, this would create a vacuum what sucks in your coolant.
            they say it works on m20 so why not on m42.

            or you can bring it to a decent auto shop, they have coolant fillers that do sort of the same thing.

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              #21
              Just to rule it out take an infrared thermometer gun and check the temp at the sender housing, if it reads fine but the gauge reads hot tap the temp gauge and see if the needle moves, if it does pull your cluster and tighten the nut on the back of the temp gauge.
              1990 325i: slippery slope from DD/DE car to SE30/ITS
              1991 318is: raw DD
              2004 330i zhp: civilized daily driver

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                #22
                The best way to bleed an M42 is to jack only the front left of the car up, not the whole front end. This puts the bleeder at the highest point and bleeding is a breeze if you do it this way. I just replaced my radiator and bled the system yesterday, it was the fourth time I've bled the system on my car this way and it has been easy every time.
                Last edited by varg; 12-10-2015, 07:10 AM.

                IG @turbovarg
                '91 318is, M20 turbo
                [CoTM: 4-18]
                '94 525iT slicktop, M50B30 + S362SX-E, 600WHP DD or bust
                - updated 1-26

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                  #23
                  So it turns out I was a dummy and it was just the fan clutch. But now I have a fresh water pump and thermostat, so that's good.

                  Thanks again for all of the help everybody.

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