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Thread: Filling and bleeding cooling system

  1. #1
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    Filling and bleeding cooling system

    I've studied the Bentley manual and the BMW repair instructions on how to fill and bleed the cooling system on my E36 M42 318is (integral expansion tank). But I'm still not clear.

    My understanding is that I'm supposed to fill with the ignition on (engine not running) and the heater running in hot. I do this is with the bleed screw open and fill until I get bubbles out of the screw. I believe this requires that I fill beyond the indicated maximum capacity right? And then I start the engine with the radiator cap still open...Is the bleed screw still open too? I then give the throttle a few blips and fill to the brim again as the level in the tank will probably drop. Then I close up the cap and the screw and run the engine up to temperature. I shut down the car, and after it cools, I check the level again and top up as necessary. This time, I shouldn't need to do any bleeding or open the screw, right?

    Please correct me where I'm wrong. Doing it this way, is there any chance that I'll end up too full and needing to drain from the expansion tank? That's what happened to me last time, although I did things slightly differently. Thanks.

  2. #2
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    I gotta admit, this is one job that's worth taking to a shop. It's just messy! Actually, most quick oil change places can do it, and it's worth it. They have a machine that they can hook up that totally flushes your cooling system, reclaims the old stuff, and leaves you ftopped off with fresh new coolant.

    Takes about 20 minutes, and maybe $40-50 bucks. It's just not worth the hassle of spilling glycol all over your garage floor, plus if you have pets they WILL drink it. That's very bad!
    The only thing new in the world, is the history that you don't know - Harry S. Truman

  3. #3
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    Hmm, it didn't seem that bad when I did it the last time though.

  4. #4
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    You need to thoroughly flush your coolant to reduce possible corrosion. "Bleeding" it the way you're describing doesn't sound like flushing. It sounds like "letting some out and putting some back in". When you flush it (or as I described above). You hook up the return hose (top one on your car) to the coolant machine. You start your engine, turn on your heater so as to open up the coolant through the heat exchanger, this assures that all of the coolant will be circulated, and let the motor run. The hot coolant will be expelled through the top hose into the machine for reclamation. Meanwhile, you fill the radiator with fresh coolant as the old is pumped through the engine. As the new coolant makes its way through the system, you will see it start coming out in the machine. It's usually a "brighter" color, much the same as knowing when you flush brake fluid, you see the fresh stuff pumping through.
    Like I said, for me I'm satisfied that it's been completely flushed out. No muss no fuss. I'm a diehard DIY kinda guy, but this is just one of the things that isn't that costly and for the mess potential, it's worth it to me.
    I have a higher mileage car (100K) and I plan on keeping it for a much longer time. I've got 3 boys that I hope to pass it down to, so good preventative maintenance is what I'm trying to do. I think a good flushin' makes us all feel better.
    The only thing new in the world, is the history that you don't know - Harry S. Truman

  5. #5
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    Yes, I do plan on flushing it and not just filling it. But I wanted to find out the proper way to fill it and bleed it at the end, and I think I've got a better idea now. Thanks.

  6. #6
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    If you have a big enough catch pan, it's not really all that messy to do. You reall do need to drain from both plugs though (engine block and radiator).

    After you fill up and the resevoir won't go down anymore, close the resevoir cap and open the bleeder screw while the engine is still running. You might have to open and close the bleeder screw a few times to make sure all the bubbles are out. Let it cool and then check the resevoir and fill as needed.

    -Bobby

  7. #7
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    One more thing on the bleed.

    After you refill it, no matter how meticulus you are, there is always going to be some air left in the system. This results in the car needing coolant added on nearly every heatup/cooldown cycle for about 4 cycles.

    My 740i took nearly 3.4 gallon of coolant in the 2 days following my radiator/water pump change out.
    Greg

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  8. #8
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    Thats 3/4 of a gallon, not 3.4 gallons
    Greg

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  9. #9
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    I did the coolant flush/change/bleed today. It wasn't too messy this time. I must be doing something slightly differently. I filled/bled twice today, once with the "cleaning" water, and once with the proper coolant mix. The first time, with water, I filled about 7.5 L, way over the capacity of 6.7 L, and I still could not get any water out of the bleed screw. I got my friend to start the car, and when he revved it up a little, a stream of water did come out. The next time, with the coolant, I filled only the suggested capacity, and this time even revving didn't do anything until he revved it really high and created a fountain that ended up wetting a corner of my engine compartment. I don't know if this actually bleeds the system or just sprays water out of the bleed screw! Anyway, I left the bleed screw open until about warm just in case any air was trapped.

  10. #10
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    When you bleed the system, you're not supposed to take the bleeder screw completely off. You just loosen it so fluid can come out and then tighten when there's no bubbles. Make sure you use a hose and wash away any coolant that comes out and don't get any on the belts.

    -Bobby

  11. #11
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    why cant you get coolant on the belts? Is it bad for them?

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  12. #12
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    Originally posted by psk145
    why cant you get coolant on the belts? Is it bad for them?
    I'd imagine it would corrode them, or at they very least make them slip.
    - Jon, '93 325is



  13. #13
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    It makes the belts slip and squeal like stuck pigs.

    -Bobby

  14. #14
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    So I wanted to top up my coolant a little bit yesterday. The engine was slightly warm but cold enough to be in the blue range. I opened the cap and was about to pour in more coolant when I decided to try opening the bleed screw a bit. I knew I didn't have to but was curious. Once I opened it, the level in the expansion tank rose a couple of inches. Did I just let air back into the system? I'm guessing that I put some air into the top of the radiator, pushing the fluid into the expansion tank? I wanted to rebleed the system so I started the car up again, coolant started seeping out of the bleed screw almost right away, so it looks like this isn't going to work. Is there anything I should do now? Thanks.

  15. #15
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    Just do a normal re-bleed. Start the engine, let it run for a bit, then open the bleeder screw. If there's no bubbles, don't worry about it. Let it cool down and re-check the coolant level in the overflow tank. Fill as needed and check again in a couple of days when it's cold.

    -Bobby

  16. #16
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    hcc... just leave the bleed screw alone. Close it tight, and don't think about it again until you want to drain the system! When you open it, you're just letting the pressure equalize, and the water return hose (top) just let loose of the water inside itself and back into your expansion tank. Thats why you think you're letting air in. You ARE! Think about it... would you bleed the bubbles out of a pop bottle from the bottom - No. You open the top and all that trapped air comes out the top.

    You're not really trapping air bubbles in your system. Your system basically purges itself of air with the first cyle of coolant thru, it goes out thru the return line into the expansion tank.

    If you want to make sure its filled to the proper level, do the following:

    Get your car up to operating temp (temp guage pointing in middle) let S-l-o-w-ly remove the radiator cap. Just crack it open to relieve the pressure, be very careful that you don't open too quickly or it could "burp" some hot coolant out (see "pop bottle analogy"- OUCH!!)

    Now start you car, and rev it up a little (1200-1500 rpm), either have someone help you, or just push the throttle linkage on the top of your motor. You will see the coolant flowing from a small opening just under the neck of your radiator down into the tank. This is from the return hose, and is normal, when at operating temperature. While its running, slowly add your coolant up to the proper level. When your done, close the cap and forget about it.
    The only thing new in the world, is the history that you don't know - Harry S. Truman

  17. #17
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    Thanks for the reply. I think my confusion comes from not understanding the coolant circuit properly. I understand where the flow comes in and goes out, etc., but I don't understand the 'link' between the radiator and integrated expansion tank. The diagrams below don't show me exactly where the bleed screw is. What I don't understand is why opening the cap alone doesn't equalize the pressure. Won't the pressure in the radiator equalize the flow once the cap is open? I know that the coolant purges itself of air as it flows back into the expansion tank, but wouldn't having excess air in there increase nominal pressure? Thanks again.




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