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    M30 Guys: Coolant expansion tank details needed

    Hey fellas,

    I've been fighting air in my cooling system for years. It's bad enough that I have to stop every 10-20mi to bleed it out, or I'll end up with a massive air pocket in the thermostat housing that will cause my temp gauges to peg and eventually puke coolant out of the expansion tank overflow line.

    The most obvious cause was a bad HG or cracked head/block that was allowing combustion gases to get in to the cooling system. However after checking for both hydrocarbons and acidification multiple times, I'm confident that this is not the cause. Fast forward after many, many sleepless nights and I think I may have found a cause.

    The thermostat housing sits roughly 2" higher than the top of the radiator (e28 535i) and at the same height as the cap on the expansion tank. However the fluid level in the tank is 2" below the cap. So while the fluid level is at the same height as the top of the radiator, it's actually 2" lower than the thermostat housing. I can bleed the thermo housing by lifting up the tank and cracking the bleed screw, but once I drive the car and the water pump circulates and it heats/cools the fluid in the system, inevitably those fluid levels will equalize and I'll have 2" worth of air gap in my thermostat housing. That's my working theory.

    I'm running the stock late model m20 expansion tank, which a quick Google image search show is the same tank being used on ~90% of the M30 swaps I can see. I see some swaps using the M30 expansion tank that mounts to the firewall, but due to the location of a bunch of relays and the strut bar, I don't have room. (side note: our LeMons/ChumpCar E30 has an M30B35 in an identical installation, except it runs the M30 expansion tank. It's never had a single cooling issue).

    So I need some detailed info and photos from you M30 guys:

    - What expansion tank are you running? M20 (early or late model) or M30?
    - Does your thermostat housing sit above your radiator? By how much? What radiator are you using?
    - How high is your thermostat housing compared to the fluid level in your expansion tank?

    I know I can't lower the front of the engine as there's only 1/8" between my oil pan and the steering rack. And I can't raise the expansion tank more than ~1/4" because the cap will hit the hood. But there must be something I'm missing... Help!

    (pics to come later today)

    #2
    M30 Guys: Coolant expansion tank details needed

    Im running the m20 expansion tank. I have an early model car so I built custom brackets for both the radiator(mishimoto e36) and the expansion tank which i out lower then my thermostat to make it the highest spot. When i bleed it i do the metric mechanic way and then run it and do it the normal way. I donít have any cooling issues except in traffic but thats because i need a bigger better fan only have one on the front of the rad. Other then in traffic my temps are always at 1/4 which Iíve found to be normal on these swaps. Im also running a colder thermostat.



    Edit my expansion tank is actually just a touch higher but radiator is lowest. But like i said no cooling issues here expect in traffic on really hot days


    1989 325is l 1984 euro 320i l 1970 2002 Racecar
    1991 318i 4dr slick top


    Euro spec 320i/Alpina B6 3.5 project(the never ending saga)
    Vintage race car revival (2002 content)
    Mtech 2 turbo restoration
    Brilliantrot slick top "build"

    Comment


      #3
      hi.

      i think my thermostat sits above level to the radiator i'll check.

      what i do is fill the system with the bleed screw open until just water comes out the top.

      tighten it and run until operating temps.
      let it cool and fill it again. Should be good to go after that.
      Don't run it without the cap or one of those fill buckets. you will lose allot of coolant and put air back in the system.

      you will get the rest of the air out by taking the cap off before first drive and check levels.

      maybe air line that goes to the tank is clogged.
      Try unclipping the tank and raising it while bending the little line do u see allot of bubbles?

      Goes without saying but just in case
      make sure the thermostat is orientated correctly because it can go in backwards, i have done this,
      and if there is a hole make sure it is pointing up.

      i am using the stock late model tank. never had a problem with air.

      only pic i can find right now but i don't think it matters where it's at?







      What have I done by jet jones, on Flickr

      Comment


        #4
        Had a similar issue with a few different cars over the years. I would definitely confirm that the recirc hose is not clogged or otherwise obstructed.

        Also, good to see you back posting on here again.

        AB
        Autobahn Motorsport - Portland, Oregon
        @autobahnmotorsport
        '88 M5 | '88 535i | '88 325i | '88 325is | '88 330is | '91 325is | '91 M-Technic 325ic | '93 740il | '03 530i

        Comment


          #5
          Originally posted by ThatOneEuroE30 View Post
          Im running the m20 expansion tank. I have an early model car so I built custom brackets for both the radiator(mishimoto e36) and the expansion tank which i out lower then my thermostat to make it the highest spot. When i bleed it i do the metric mechanic way and then run it and do it the normal way. I don’t have any cooling issues except in traffic but thats because i need a bigger better fan only have one on the front of the rad. Other then in traffic my temps are always at 1/4 which I’ve found to be normal on these swaps. Im also running a colder thermostat.

          Edit my expansion tank is actually just a touch higher but radiator is lowest. But like i said no cooling issues here expect in traffic on really hot days
          Thanks for the quick response! So if I'm understanding you, in order of lowest -> highest, your car is radiator -> thermostat -> expansion tank, yes? Are you running the stock vent line from the radiator to expansion tank?

          What is the "Metric Mechanic" was of bleeding? I'm not familiar with that. Also, how are you controlling your fan? Factory style temp switch?

          Originally posted by LowR3V'in View Post
          hi.

          i think my thermostat sits above level to the radiator i'll check.

          what i do is fill the system with the bleed screw open until just water comes out the top.

          tighten it and run until operating temps.
          let it cool and fill it again. Should be good to go after that.
          Don't run it without the cap or one of those fill buckets. you will lose allot of coolant and put air back in the system.

          you will get the rest of the air out by taking the cap off before first drive and check levels.

          maybe air line that goes to the tank is clogged.
          Try unclipping the tank and raising it while bending the little line do u see allot of bubbles?

          Goes without saying but just in case
          make sure the thermostat is orientated correctly because it can go in backwards, i have done this,
          and if there is a hole make sure it is pointing up.

          i am using the stock late model tank. never had a problem with air.

          only pic i can find right now but i don't think it matters where it's at?
          I've filled it like that before and it always seems to work perfectly, I get all the air out. But then I go for a drive and after 10-20mi my temp gauges often start to jump up because they're sensing air. Sometimes I can get the bubble to push through to the radiator if I really floor it (remember I'm having to push air down 2") and then I think it gets out to the expansion tank via the vent line.

          I also have a fluid extractor with a coolant vacuum fill setup that I've used. According to everything I've read about these, this is the absolute best way to fill and bleed a system. But I get the same results: perfect initial bleed, then continuously getting air as I drive it.

          Originally posted by e30austin View Post
          Had a similar issue with a few different cars over the years. I would definitely confirm that the recirc hose is not clogged or otherwise obstructed.

          Also, good to see you back posting on here again.

          AB
          Ya, my last login was almost a year ago! Family tends to change priorities. I'll check the line, and I think it's worth replacing the expansion tank cap as well.

          Comment


            #6
            Originally posted by CorvallisBMW View Post
            Thanks for the quick response! So if I'm understanding you, in order of lowest -> highest, your car is radiator -> thermostat -> expansion tank, yes? Are you running the stock vent line from the radiator to expansion tank?

            What is the "Metric Mechanic" was of bleeding? I'm not familiar with that. Also, how are you controlling your fan? Factory style temp switch?



            I've filled it like that before and it always seems to work perfectly, I get all the air out. But then I go for a drive and after 10-20mi my temp gauges often start to jump up because they're sensing air. Sometimes I can get the bubble to push through to the radiator if I really floor it (remember I'm having to push air down 2") and then I think it gets out to the expansion tank via the vent line.

            I also have a fluid extractor with a coolant vacuum fill setup that I've used. According to everything I've read about these, this is the absolute best way to fill and bleed a system. But I get the same results: perfect initial bleed, then continuously getting air as I drive it.



            Ya, my last login was almost a year ago! Family tends to change priorities. I'll check the line, and I think it's worth replacing the expansion tank cap as well.


            Yes radiator is lowest, then thermostat is next lowest. Expansion tank is highest. Using stock vent line. Metric mechanics way of bleeding is disconnect the vent line from expansion tank and hold it up as high as possible attach a new line to the tank and with the cap on blow into the new line until coolant comes out of the line youíre holding up. This pushes the air out giving you a bled system. I have fan switch inside my car i flip it on high when im in traffic and flip it off when cruising


            1989 325is l 1984 euro 320i l 1970 2002 Racecar
            1991 318i 4dr slick top


            Euro spec 320i/Alpina B6 3.5 project(the never ending saga)
            Vintage race car revival (2002 content)
            Mtech 2 turbo restoration
            Brilliantrot slick top "build"

            Comment


              #7
              Rad level by jet jones, on Flickr


              please recheck thermostat is in correctly.

              Comment


                #8
                Originally posted by ThatOneEuroE30 View Post
                Yes radiator is lowest, then thermostat is next lowest. Expansion tank is highest. Using stock vent line. Metric mechanics way of bleeding is disconnect the vent line from expansion tank and hold it up as high as possible attach a new line to the tank and with the cap on blow into the new line until coolant comes out of the line you’re holding up. This pushes the air out giving you a bled system. I have fan switch inside my car i flip it on high when im in traffic and flip it off when cruising
                Interesting. I can see how that would push air out of the system as long as it was all collected in the radiator (where the vent line is). My problem is that the radiator is lower than the thermostat, and air doesn't like to go downhill. But I suppose doing the same 'blow in to the expansion tank' while cracking the bleed screw would address that issue. Essentially it's exactly the same method as raising the expansion tank to bleed.

                Originally posted by LowR3V'in View Post

                please recheck thermostat is in correctly.
                I will check that just in case, yes. It's not a problem of the car overheating, it's just the damned air! Your radiator definitely sits higher than mine; it's clearly above the thermostat. Maybe I need to consider moving mine up if possible.

                Here's an elevation sketch of my system. You can clearly see that filling the expansion tank to the factory cold coolant level will mean leaving an air gap in the thermostat housing. I can purge that air gap by lifting the expansion tank and opening the bleed screw, but once I reinstall the tank the head pressures will equalize and eventually so will the coolant levels. I'm starting to think my best course forward is to find some way of raising the tank, but I've got only about 1/2" between the cap and the underside of the hood.

                Capture by Darin Schnoor, on Flickr

                And here's what the actual elevations look like. Radiator is about 1.5" below the top of the thermo housing:
                IMG_0155 by Darin Schnoor, on Flickr

                And the top of the thermo housing and top of the expansion tank are dead even:
                IMG_0156 by Darin Schnoor, on Flickr

                And since the cold coolant level sits about 1.5" below the top of the tank, the coolant level and top of the radiator are essentially at the same exact height, with the thermo being 1.5" higher. 1.5" may not sound like a lot, but that's half the diameter of the thermostat, meaning it's only half submerged and not going to operate properly. I also means the temp sensors are going to be immersed in air, not coolant.
                Last edited by CorvallisBMW; 06-04-2019, 09:21 AM.

                Comment


                  #9
                  Once a cooling system is bled, the location of the components doesn't really matter.

                  Are you bleeding the system with the heater fully on (valve open) ?
                  Lorin


                  Originally posted by slammin.e28
                  The M30 is God's engine.

                  Comment


                    #10
                    so when it over heats just let it cool down and crank the bleed screw
                    and fill up. Even when i blew my head gasket i just put a gal back in and drove home without it overheating.
                    bend and wiggle the little hose going to the tank while filling it up i think there is something just clogged or backwards
                    Last edited by LowR3V'in; 06-04-2019, 09:15 AM.

                    Comment


                      #11
                      Originally posted by LJ851 View Post
                      Once a cooling system is bled, the location of the components doesn't really matter.

                      Are you bleeding the system with the heater fully on (valve open) ?
                      How does it not matter? Fluids always flow to the lowest point. And in a system with equalized pressures, the fluid levels would always equalize, right?

                      Yes I always bleed with the heater on high and the key in pos. 2

                      Originally posted by LowR3V'in View Post
                      so when it over heats just let it cool down and crank the bleed screw
                      and fill up. Even when i blew my head gasket i just put a gal back in and drove home without it overheating.
                      bend and wiggle the little hose going to the tank while filling it up i think there is something just clogged or backwards
                      Yes, I've opened the bleed screw to purge the air literally dozens of times. I always get air out, then I drive for another 10-20 miles, and get more air. I can do this ad nasuem from now until eternity. I've replaced the vent line with 100% new hose and verified it flows correctly. But as I've said above, the vent line is BELOW the thermostat housing. So if air gets trapped in the thermostat housing, the vent line won't purge it.
                      Last edited by CorvallisBMW; 06-04-2019, 10:31 AM.

                      Comment


                        #12
                        Sometimes, what I've done, is remove the hose to the expansion tank to radiator (at the tank), place it in a container and fill the cooling system (overflowing, obviously) and allow whatever air/aeration to pass. M42 radiators usually require something similar to this, as they are a real pain in the ass to bleed out. I'm sure you fill your cooling system through the upper hose as well, yes?
                        Autobahn Motorsport - Portland, Oregon
                        @autobahnmotorsport
                        '88 M5 | '88 535i | '88 325i | '88 325is | '88 330is | '91 325is | '91 M-Technic 325ic | '93 740il | '03 530i

                        Comment


                          #13
                          Originally posted by e30austin View Post
                          Sometimes, what I've done, is remove the hose to the expansion tank to radiator (at the tank), place it in a container and fill the cooling system (overflowing, obviously) and allow whatever air/aeration to pass. M42 radiators usually require something similar to this, as they are a real pain in the ass to bleed out.
                          OK, I'm trying to picture this. The vent line is running from the rad to a container and the engine is running and the coolant is overflowing from the open expansion tank cap, yes? What makes this different than leaving the vent line attached to the tank and letting it purge in to there?

                          Originally posted by e30austin View Post
                          I'm sure you fill your cooling system through the upper hose as well, yes?
                          I haven't tried this method in a long time, as it only seems to fill the radiator. If the thermostat is closed coolant can't move through and in to the engine, yes? Lately I've been filling via a vacuum filler like this, which is supposedly the penultimate method: https://www.ecstuning.com/b-schwaben.../024948sch01a/

                          Comment


                            #14
                            Originally posted by CorvallisBMW View Post
                            How does it not matter? Fluids always flow to the lowest point. And in a system with equalized pressures, the fluid levels would always equalize, right?

                            Yes I always bleed with the heater on high and the key in pos. 2

                            Once the air is fully gone its all fluid inside the cooling system, the only air is at the top of the expansion tank. That air is not going to go down through coolant in the tank and into the engine to equalize levels.

                            As an example, on one of my motorcycles i have the remote brake fluid reservoir completely below the clutch and brake master cylinders it feeds. The air on top of the fluid in the reservoir does not go down through the fluid and back up to the master cylinders and the fluid does not drain out of the master cylinders into the reservoir. Think a straw full of fluid with your finger on top of it.


                            I think your system is not fully bled or creating new air, or has a leak somewhere .
                            Lorin


                            Originally posted by slammin.e28
                            The M30 is God's engine.

                            Comment


                              #15
                              Originally posted by LJ851 View Post
                              Once the air is fully gone its all fluid inside the cooling system, the only air is at the top of the expansion tank. That air is not going to go down through coolant in the tank and into the engine to equalize levels.

                              As an example, on one of my motorcycles i have the remote brake fluid reservoir completely below the clutch and brake master cylinders it feeds. The air on top of the fluid in the reservoir does not go down through the fluid and back up to the master cylinders and the fluid does not drain out of the master cylinders into the reservoir. Think a straw full of fluid with your finger on top of it.


                              I think your system is not fully bled or creating new air, or has a leak somewhere .
                              So here's my thinking on this: It's not that the air is going down through anywhere, it's that the coolant is pushing up. If there's 1.5" of fluid in the thermostat above the fluid in the expansion tank, that fluid is going to exert a downward force (called head pressure). It will push, albeit with a fairly small force, through the coolant passages of the engine and in to the expansion tank until the levels equalize. Meanwhile the air that's trapped in the expansion tank is going to push backwards through the vent line and in to the top of the radiator (this is what's different between my system and your bike; your reservoir is sealed, so the air cannot go anywhere. In mine, that vent line allows the air to move, which in turn allows the fluid to move. It's a straw with a finger only partially covering the top. If you drilled a tiny hole in your reservoir cap, the fluid would backflow down the line. Maybe I need to cap off my vent line???). Now with the engine off all of the pressures are equalized and the only force acting on the system is gravity. As such the fluid level is going to try and equalize itself everywhere in the system. Therefore the only way to keep air out of the engine is to ensure the fluid level in the expansion tank is at least as high as that point or higher.

                              Edit: Here's a simplified version of the diagram. The fluid levels will always equalize as long as the pressure is allowed to flow from one container to another. If I raise the expansion tank, the fluid level in it will drop as it moves to the engine. But as soon as it goes back to it's normal position, everything will level out. However if I were to block the vent line, that would prevent the pressures from equalizing, and then the fluid could be kept at dissimilar levels.
                              Capture1 by Darin Schnoor, on Flickr
                              Last edited by CorvallisBMW; 06-04-2019, 12:27 PM.

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