Captive cooling system

Discussion in 'The Big Chill' started by Gulfgears, May 19, 2020.

  1. Gulfgears

    Gulfgears Gulfgears

    I was checking the radiator fluid level yesterday and noticed it was down about a pint, low enough that the cooling fins for the transmission were visible.

    This seems to be an on going problem, I thought the aluminum radiator I put in a couple of years ago would cure it, but no. when car had stock 3 core radiator it would always be well below the filler neck too.

    Essentials: Stock 300 with ST transmission
    Timing is correct, and no spark knock.
    Using original fan shroud and stock thermo fan (which blows very hard)
    Have a 16# radiator cap (Stant) per the manual
    Runs cool most of the time, around town with ac on it can get excited and go over towards 200
    Running anti freeze and a can of be cool stuff

    I see ads for coolant recover systems, but it never explains how the fluid gets sucked back into the radiator past the radiator cap. Most appear to be nothing but catch cans.

    Goal is to have a system like on my more modern cars that is sealed and lets the fluid level rise and fall as needed.

    Is this possible?

    Going to get a new radiator cap just in case the old one might be the culprit, but I highly doubt it.

    Tried to find somebody to check the existing cap but no one seems to have a tester

    Car does not leak any antifreeze anywhere and nothing in the oil like water.

    Oh, have 165 degree thermostat in.

    Thanks for any ideas.
  2. mrolds69

    mrolds69 "The Cure"

    Gary, old systems are not like new systems. On a regular old car system, if you take the cap off (cold) the coolant should be about an inch or a bit more low. If your radiator is original, it will say on the side how high it should be. I forget what it says, full or cold, or what. Then the car runs, it expands, if it's over full it discharges. Newer systems use a different cap with 2 gaskets, and like a one way valve or something to keep the radiator full to the top. You can buy an old NOS universal system off like ebay, or at a swap meet. It will come with a bottle, bracket, and new special cap. But, it's normal to be down an inch or more with no bottle. My 67 GS400 is like that. If you buy an aftermarket deal, make sure it includes the cap.
  3. mrolds69

    mrolds69 "The Cure"

    Hmmm, I don't see any. Maybe they are HTF now. You can retrofit the 70-72 type tank on your car. You still would need a later closed system type cap, you would need to clamp the overflow hose, etc.
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The 70's cars have coolant recovery systems. I don't see why you can't use them on an older car, all you need is a recovery container. The radiator cap just keeps the system at a certain pressure. Expanding coolant has enough pressure to unseat the seal on the cap and vent the extra coolant into the tank. When you shut down the engine, heat soak into the coolant increases the pressure in the system, and as it cools, some coolant is sucked back into the radiator until the cap seals when the pressure falls below the cap rating. Then the radiator remains full.
    GS464 likes this.
  5. FJM568

    FJM568 Well-Known Member

    I've got one similar to this one on my 1966 Skylark...


    And then I bought a radiator cap from a later model GM car that had the closed recovery system. Your hose from the overflow of your radiator will go to the hose nipple that is the shorter one on the bottom of the tank(it only extends just into the bottom of the tank). The other tube out the bottom is a tall tube that goes almost all the way on the inside to the top of the container, a short hose goes on the bottom of that to direct the overflow on the ground if you should overheat it enough.

    I ended up bead blasting mine to rough it up and painted it black and mounted it behind my grill on the core support in front of my radiator (actually to the side in front of radiator)

    Attached Files:

    Last edited: May 19, 2020
  6. Gulfgears

    Gulfgears Gulfgears

    Thank you all for the info.
    Do you happen to recall what year/make you got the cap from, or does it make a difference as long as it's a 16# rating?
    The can you showed was what I had in mind as a catch can.
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Doesn't make any difference. Take a look over at Rockauto. Look up the cap for a 1966 Special with a 300. Then look up a 1970 GS455. Same caps.
    Last edited: May 20, 2020
  8. Electra Bob

    Electra Bob Well-Known Member

    I recently replaced the stock recovery/expansion tank on my car with one from Holley.

    The way this Holly tank is designed is for a top feed through the side barb connector and to blow off excess through the cap... yah, no. I modified it by fitting a 90 barb connector where the drain plug is and removing the interior plastic pipe from the top connector. I then ran a hose from the radiator overflow connector to the base and another hose from the top connector down towards the ground. This then works in the same manner as the stock tank albeit with slightly less capacity.

    I initially filled it to about half way. When the engine got up to full working temp it pushed fluid into the tank until some was forced out the overflow. As it cooled down it drew coolant back into the radiator until it reached the level shown in the picture below. So now it it will fill the tank when hot and draw back into the rad when it cools.

    The radiator is a drop in replacement from Champion

    mrolds69 likes this.
  9. mrolds69

    mrolds69 "The Cure"

    I see where that's working for you, Bob. It's a nice set up. You don't think there should be clamps on the rubber hose?
    Last edited: May 20, 2020
  10. Electra Bob

    Electra Bob Well-Known Member

    It's essentially a vented system so no pressure, just a flow of coolant to and from the radiator.

    I know the stock system has clamps but the hose I used is pretty tight on the barb connectors.

    mrolds69 and Gulfgears like this.
  11. Gulfgears

    Gulfgears Gulfgears

    I always hate it when the original poster never advises how, or if the problem was solved.
    So, here's the latest and hopefully the final chapter in my cooling problems.
    I went to NAPA today and told the counter man I wanted his best radiator cap, thinking this is NAPA so this could really hurt!
    He came back with a Belknap cap, said this one is their most popular one and it works well.
    Much to my surprise it only cost $6.00.
    Took it home and immediately noticed that it fit much tighter than the cap I had on there, and seemed like a more substantial piece.

    Made sure radiator was full to just below the filler neck. Then jury rigged a plastic bottle to serve as a catch can and attached it to the overflow tube.

    Cranked up the car, turned on the a/c went for a drive (found a barber shop way out in the country, no waiting). Temperature never got above 180 with outside temp of around 90. Then went to what passes for our more urban area here and sat in traffic for awhile (what happened to stay at home?). Temperature climbed to just below 200 but immediately dropped to around 170 when started moving.

    Got home, let the car cool down and checked radiator, fill level was perfect, and there was no fluid in the bottle. Remember this is car that used to puke everytime it was parked.

    So in my case it seems to be a defective radiator cap. Hope that truely is what the problem was.

    Got a neat surprise as I was blasting down the interstate at 75-80 and the oil pressure light came on. I have both the idiot light and a analog pressure gauge. Pressure was fine, appearently got a bad connection on the idiot light.

    So there it is one problem solved now on to making the AFB I have start behaving.
    mrolds69 likes this.
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    In order for the overflow container to work, the radiator has to be filled as much as possible. Then you will get fluid expansion into the tank. What i do is open the throttle when I get close to the top. That temporarily depresses the level. I then hold the throttle with one hand, fill the radiator to the top, put the container down, and put the cap on, then let go of the throttle. Then put some coolant into the overflow. One or two drives and the air is purged from the overflow tube, and the system works.
    GS464 likes this.
  13. Electra Bob

    Electra Bob Well-Known Member

    Putting the car up on ramps also puts the radiator at a high point above the engine. Then, with the radiator cap off. Top up the radiator, give the top hose a slow squeeze to bubble out some air then let it go quickly and it will suck coolant into the hose. Top off the radiator and repeat until there is no gain.


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