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M50 cooling fine at idle - Overheats immediately when driving - Pressure in Radiator

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    M50 cooling fine at idle - Overheats immediately when driving - Pressure in Radiator

    I have been chasing down this cooling issue for a while now, hopefully someone can shed light on my situation. I picked up the car at the beginning of this year with the M50 swapped in, I’ve been basically cleaning things up since.

    This first started when I noticed the radiator was getting pressurized at startup. Used a block tester to check for HC in the coolant and came up with nothing, so I ended up getting a new (M42) radiator and t-stat for the car. Once that was in it ran/cooled great. After about 3 weeks of driving, it randomly started wanting to overheat.


    Symptoms:

    If I let the car sit and idle, it comes up to temp normally and then climbs slowly like you would imagine with no fan on and the car parked. I can basically let it sit idling for 15 minutes and it is fine. Now if I take the car down the road, as soon as it gets up to temp it gradually climbs in temp until I shut it off. If I start the car without the radiator cap on, it blows all of the coolant right out of the reservoir. Oddly enough, once the pressure is relieved, when using the block tester it appears to be creating a vacuum in the reservoir tank. Just recently in this whole testing process it has started to blow pressure out of the cap, even if cold and I only run it to pull out of the garage or something.


    Here’s what I have tried:

    -Swapped out thermostat – Current one is a alternate temp one and has the bleeder hole
    -Bled the system multiple times (on a hill, also tried a burp bucket a few times)
    -Compression test (180psi across the board, 2 cylinders lower but within 10psi)
    -Flushed the coolant from the block drain
    -Used a block tester to check for hydrocarbons, both through the reservoir and the bleeder hole (all tests negative)
    -Oil doesn’t look milky at all. Under the filler cap is clean as well.

    Pressure in the cooling system when the car is cold typically means head gasket, but I don’t see how that would be the case and yet I still get negative results when I test for hydrocarbons in the coolant.

    If the HG is bad, I want to be able to prove it 100% before I tear the car apart to fix it. Any ideas what else this could be?

    #2
    How are you bleeding the car? Have you tried raising the front of the car to ensure the bleed screw is the highest point? You could have air pockets in your block.

    Comment


      #3
      Originally posted by earthwormjim View Post
      How are you bleeding the car? Have you tried raising the front of the car to ensure the bleed screw is the highest point? You could have air pockets in your block.
      This is what I thought initially. I have tried bleeding it on a hill a couple of times. I have also tried a burp bucket, which emulates the same thing essentially, making the bucket the highest point in the system. No matter how much I bleed it air remains in the system.

      Comment


        #4
        Sounds like you have failed head gasket or slightly crooked head. I have encountered that also. Everything seems fine when the car is idle. No overheating, no exhaust gasses in coolant, compression test ok etc. But immediately when you start driving, there is too much pressure in cooling system and the engine overheats. That's because the head gasget still holds when the engine is at idle but it starts to leak into cooling system after you get some load to the engine.

        In this case I had same symptoms and as you can see it has leaked exhaust gasses into coolant system from those two cylinders:


        Just change the gasket and resurface the head and you will be ok.

        Comment


          #5
          Originally posted by pazi88 View Post
          Sounds like you have failed head gasket or slightly crooked head. I have encountered that also. Everything seems fine when the car is idle. No overheating, no exhaust gasses in coolant, compression test ok etc. But immediately when you start driving, there is too much pressure in cooling system and the engine overheats. That's because the head gasget still holds when the engine is at idle but it starts to leak into cooling system after you get some load to the engine.

          In this case I had same symptoms and as you can see it has leaked exhaust gasses into coolant system from those two cylinders:


          Just change the gasket and resurface the head and you will be ok.
          In your case were you still getting bubbles in the coolant at idle? That is my only hangup here. I am clearly getting air coming out of the reservoir, yet it tests negative for exhaust. One would think those air bubbles are being generated by the compression chamber, thus would have exhaust gasses present.

          The rest of it sounds consistent to what I'm experiencing. I just want to be 100% sure. I would hate to tear the head off only to find out that is not the issue.

          Comment


            #6
            Originally posted by Chariot View Post
            In your case were you still getting bubbles in the coolant at idle? That is my only hangup here. I am clearly getting air coming out of the reservoir, yet it tests negative for exhaust. One would think those air bubbles are being generated by the compression chamber, thus would have exhaust gasses present.

            The rest of it sounds consistent to what I'm experiencing. I just want to be 100% sure. I would hate to tear the head off only to find out that is not the issue.
            I don't remember exactly. But there is one thing that you could check. Is the coolant hose from the back of the head going to the lower pipe for the interior heater element in firewall. Because if it's connected to the upper one, then the coolant system is almost impossible to bleed. There is always air trapped inside the interior heater element. But that shouldn't cause the problems you are having.

            Comment


              #7
              Replace the coolant with liquid nitrogen. That should keep the temp gauge where you want it.

              Comment


                #8
                Originally posted by pazi88 View Post
                I don't remember exactly. But there is one thing that you could check. Is the coolant hose from the back of the head going to the lower pipe for the interior heater element in firewall. Because if it's connected to the upper one, then the coolant system is almost impossible to bleed. There is always air trapped inside the interior heater element. But that shouldn't cause the problems you are having.
                The coolant hose coming from the back of the head (basically right above where the transmission mounts to the block) is connected to the upper heater core pipe on my car. Clearly that is an issue. It looks like I will need to remove the intake manifold in order to access those cooling lines to swap them.

                The next step will be to swap the lines, reassemble, bleed it, and see where it gets me.

                Comment


                  #9
                  yeah you need to swap those hoses. Normally people put those wrong way around because they go easier like that :D including me

                  Comment


                    #10
                    What pressure radiator cap are you using? The stock M42 radiator cap, at least from my 318is, is only rated to 1.4 bar I believe, you need to upgrade to a 2.0 for a swapped engine. I ran into this issue during my first swap. Might want to give this a try before delving too much further into things.
                    M/S52 Wiring Diagrams
                    Z3M S52 to late model 325i wiring diagram
                    M52 to 318is wiring diagram

                    Comment


                      #11
                      Originally posted by luckysnafu View Post
                      What pressure radiator cap are you using? The stock M42 radiator cap, at least from my 318is, is only rated to 1.4 bar I believe, you need to upgrade to a 2.0 for a swapped engine. I ran into this issue during my first swap. Might want to give this a try before delving too much further into things.
                      Do you know off hand what cap interchanges? I suppose I could check out the one on my e46 and see if that would work. I'm almost positive I'm using the stock M42 cap. Some research shows that the part number I need would be (17 11 1 742 231).

                      Even if I have a 2bar cap on there now I will just buy a new one. I'm hoping to have the intake pulled off this weekend so I can swap the cooling lines. Hopefully after that and a new cap this damn thing will work again.

                      If not...next step is head gasket.

                      Comment


                        #12
                        I'm pretty sure any e36 or e46 radiator cap will work, as long as it's rated 2 bar
                        M/S52 Wiring Diagrams
                        Z3M S52 to late model 325i wiring diagram
                        M52 to 318is wiring diagram

                        Comment

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