Should radiator draw coolant back from overflow?

Discussion in 'The Garage' started by Mastiff, Aug 1, 2020.

  1. Mastiff

    Mastiff 1/2 ton status Premium Member

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    My understanding is that when the coolant heats up, and the radiator cap exceeds releasing pressure, coolant will flow into the overflow. When the system later cools down, it ought to create suction and draw coolant back into the radiator from the overflow. On my 86 though, the coolant always ends up like an inch down or so from the top. I don't think this itself is a big deal, but should it really be able to stay totally full if the system is working right? There is coolant in the overflow, the cap is new and the tubing is in good condition.
     
  2. Kain

    Kain 1/2 ton status Premium Member

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    yes it should , some of the aftermarket ones are not that great and dont.
    pull your oil fill is there a lot of condensation
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    I'm assuming the radiator is low when you pull the cap? Make sure there isnt a bunch of crud in the overflow tank at the bottom, and that the hose/nipple arent plugged somehow. One off, but once my sister used stops leak in her cooling system. Then it started puking coolant. Sure enough, the sediment (more like flakes as I recall) from the stops leak would allow coolant to get into the tank, but would prevent outflow.

    It has been hard to get quality radiator caps for a long tune now, when I find one that works I run it until it fails, regardless of what else I change. Same as thermostats unfortunately.
     
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  4. Mastiff

    Mastiff 1/2 ton status Premium Member

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    Yeah, it's just a hair low in the radiator. It sure seems that it must be drawing some back up since it's not very low. My old Scout only puked coolant out one way and the radiator had to be quite low to reach equilibrium. I'll check for crud, it's certainly possible.
     
  5. Kain

    Kain 1/2 ton status Premium Member

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    thats one thing ive noticed too the caps and stats are failing more often
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    If you have another cap to try, it wouldn't hurt to try, of course assuming the hose and tank are clear of junk.

    Maybe I misunderstood. Are you saying the coolant level in radiator is an inch under the cap? That would be incorrect.

    *Theoretically* you don't ever need to top off the coolant in the radiator. As long as there is coolant in the reservoir, the coolant level should remain roughly at the bottom of the cap, since any excess coolant should be pushed out the nipple to the reservoir.

    If the radiator is low, there should be no coolant in the overflow.
     
  7. Mastiff

    Mastiff 1/2 ton status Premium Member

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    I have coolant in the overflow and coolant an inch below cap in radiator. Likely not being drawn back fully for whatever reason.
     
  8. Kain

    Kain 1/2 ton status Premium Member

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    is your overflow cracked?
     
  9. ZooMad75

    ZooMad75 #crawlabago Premium Member GMOTM Winner

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    The radiator has to be full for the push and pull of coolant to happen right. Otherwise the coolant only expands in the radiator and never pushes into the overflow. On the same point if the radiator is low, when the coolant cools off and contracts in size it just drops the level lower in the radiator.
     
  10. Mastiff

    Mastiff 1/2 ton status Premium Member

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    I don't think so. It has plenty in there, and the hose attachment is below the fluid level.
     
  11. Kain

    Kain 1/2 ton status Premium Member

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    my rad did this there was a crack in it when i replaced the overflow bucket with one from lmc it stoped
     
  12. imiceman44

    imiceman44 Hoarder extraordinaire Premium Member

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    If the level when cold is 1" under the cap, try filling it back up cold and cycle it one more time to see if it gets better.
    Mine is like this, and I believe it is because when it's sucking in the coolant it is not completely cold and when it cools down to minimum temperature it will contract a little bit.
    I had problem before with coolant never coming back and I changed the cap and now it works fine but it is a little below the cap.
     

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