Engine Overflow Tank

Discussion in '1994 - 1995 Specific Tech' started by ace940, Oct 30, 2012.


  1. ace940

    ace940 Member

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    i bought a universal radiator overflow tank at my swap meet last spring and was wondering why its always empty. i fill it up and then the next time i check it its empty again. the radiator has plenty of water in it always. is it just flowing over and dumping through the drain. and another question i have is what is the point of having radiator caps that will release with enough pressure? the pressure should drain out of the overflow tank. the tank holds one quart. thanks
  2. jcgafford

    jcgafford Well-Known Member

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    coolant does not just disappear, it has to be going somewhere.either an internal or external leak. if it were me i would pressure check the cooling system and cap and perhaps do a block test to look for products of combustion in the cooling system.
    as for the radiator cap, it is a safety thing and allows for the fluctuation in pressure when that engine takes coolant from 50 degrees and cooks it to 180. if it didn't vent it to the overflow, or even to the air when a major boo boo happens, it would just build that pressure till it found the weakest point in the cooling system and blow that instead, which probably would be a coolant hose...
  3. JordanB21

    JordanB21 Active Member

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    You could do what I did and just watch the car for 10 minutes while hot and see if it over flows or not. It overflows around 250+.

    course that's the lazy way. do what he said above:p
  4. ace940

    ace940 Member

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    but what im saying is that it would just vent out of the overflow tank anyways so why would it matter if the cap can release pressure or not? it is going to vent anyways. and i know that im not leaking or burning coolant so it must be overflowing when it gets hot and expands. and i didnt say it was dissappearing, what i meant was do i need a bigger tank for the car than 1 qt. i know that it is flowing over i just wanted some input on the tank size if its wrong.
  5. jcgafford

    jcgafford Well-Known Member

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    the radiator cap will allow a larger amount of pressure to release than the 1/4" i.d. tube that vents to the overflow. this is so it can release that pressure before something horrible happens. if your car is venting constantly to the overflow tank then the cap is probably bad. if it is just pushing the normal amount of fluid into the overflow and the overflow is too small then eventually it would lose fluid. i believe the factory overflow is larger than one quart and probably that size for that reason.
  6. Chythar

    Chythar Well-Known Member

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    Something doesn't sound right here. The radiator cap is what allows coolant to flow to the overflow reservoir on the 94-95 Mustangs. Our radiators have a stock 16psi cap. Once the coolant reaches 16psi, the spring in the cap compresses and exposes the hole that leads to the overflow, allowing coolant to flow to the overflow tank. When the pressure drops, a vacuum is created that sucks coolant back into the radiator from the overflow tank until the pressure drops below 16psi, then the spring in the radiator cap pushes down and closes off the hole leading to the overflow tank.

    Coolant venting to the overflow tank is normal. There are markings on it showing "COLD" and "HOT", showing about how much coolant should be in the reservoir when the engine is cold or hot. If you overfill the reservoir when cold, you may get coolant pushed out of the reservoir when it's hot. You can also puke a bit of coolant out of the reservoir if your car is really hot and you shut off the car. Since the car is not moving the coolant continues to heat up for a while and in some cases can overflow the reservoir. Otherwise, the only time you'll overflow the reservoir is when you have overheated the engine.

    Other cars may have a different system, but this is how our Mustangs work.
  7. Chythar

    Chythar Well-Known Member

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    The coolant is going somewhere. Are you sure the universal tank isn't leaking? Even a small leak will drain the reservoir over time. I still have my stock plastic one and it's got a small crack in it, causing the overflow tank to eventually drain out. It's also possible that your coolant system isn't holding 16psi of pressure. If some part of the coolant system can't hold as much pressure as the radiator cap, it's going to leak coolant. In most cases, it's a hose clamp that isn't tight enough. On a Mustang I used to own, I pressure tested it and found several leaks. It was like whack-a-mole with coolant leaks - I'd pressurize the system to 16 psi, and hear coolant peeing out from a hose. Tighten the clamp, re-pressurize, and find ANOTHER hose clamp leaking! Fortunately in my case, it was all loose hose clamps.
  8. jcgafford

    jcgafford Well-Known Member

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    i agree. that is how it is supposed to work on a closed system. yet the problem still is there. either he has a bad cap or a leak somewhere. we have all watched the huge plumes of steam from under the hood of an overheated vehicle. that steam is coming from the cap, not the overflow. the cap allowed that release of pressure.

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