How much fluid in the coolant overflow reservoir?

Posted by Silver71, Aug 29, 2012
Maintenance & Repairs - Oil choices, timing belt, setting timing, CV boot replacement, alternator servicing, fuse/relay checks, and other basic maintenance, repair and diagnosis discussions.

  1. Silver71

    Silver71 Proven Member

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    So I was a little shocked when looking at my engine bay today... I noticed this little reservoir with a cap that had a hose coming in and a hose coming out of this cap. First of all, the cap was off the tank (WTFfff?). Second, one of the hoses was simply hanging there not attached to anything (double WTFfff!??)

    Deep breaths...come to DSMForums to figure things out.

    I found out that it is the coolant/antifreeze overflow reservoir. I also found out that it exists because of the fluctuation in the coolant level due to expansion and compression due to temperature differences. I also found out that it is not totally uncommon for the cap to be off due to the system overflowing. However, I have a few questions:

    Maybe someone forgot to put the cap back on after a checkup and there's nothing to worry about. However, if the reservoir did in fact overflow and blow the cap off, why would this happen, and what should I take a look at in order to rectify the problem?

    Next, If the hose dangling there is for draining excess coolant, why would the cap ever blow off at all?

    Also, a general coolant question. For my 95 GSX, what coolant specifically am I to use in the system? What color should it be normally, and what color is it when worn?

    Finally, how much coolant should be in the reservoir itself when the engine is cool? I don't see marks to indicate proper levels.

    Oh, this will sound really stupid, but where the heck is my radiator cap? I don't see it on the radiator..........:hmm:
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  2. NHerron

    NHerron Proven Member

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    Nice research :thumb:
    When the engine is cold, the coolant is ambient temperature and up to level in the cooling system. Now engine starts, and coolant starts to warm up in a closed system. When the coolant heats up, it expands and takes up more space, therefore putting equal pressure throughout this closed system. When the pressure is high enough, it gets released via the radiator cap to keep a regulated 13psi.
    Now engine is shut off, coolant slowly cools off to ambient temperatures. When the coolant cools off, it contracts and takes up less space in a closed system. It now needs additional coolant to keep the radiator up to level. So a vacuum (or lower than ambient pressure) is created, thus drawing in coolant from the overflow bottle. There is a separate valve for this, it is the little shiny disc underneath the radiator cap. You can tilt this valve open with your fingernail, it has hardly any spring pressure behind it.

    I wouldn't worry about it. Just put the cap back on and check under the hood after the next time you drive, and see if it popped again.

    IF the cap popped off from otherwise human contact, it could be that the cap isn't venting fast enough for the coolant to escape. Next time, take a look at the size of the hole the cap has. The hose is huge but the vent hole is much smaller. Blow through a coffee straw as hard as you can without pinching the straw with your lips to keep it from becoming airborne ;)

    Ethylene Glycol, it's the most common coolant out there Walmart will have it. The coolant won't necessarily change color when worn. If it has indeed changed color, then there is rust in the system or a blown headgasket. It should always stay fluorescent green.

    When the engine is cold, just fill the bottle halfway. At least keep the level ABOVE the rubber hose inside the bottle at all times.

    Picture attached. Red arrow is radiator cap.

    Most cars, just follow to biggest and topmost rubber hose. It usually leads to the cap. However on some cars (newer usually) this doesn't hold true. But then again, newer cars are so color-coded and idiot proof....even a caveman can do it!

    Attached Files:

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  3. Silver71

    Silver71 Proven Member

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    Sartell, Minnesota
    Thanks for all the info.

    I took of the radiator cap and the coolant was very green and clean (I even took some out with a pipette to look through it). I checked the pressure relief valve under the cap and it opened and closed freely.

    I was just concerned because the level in the overflow reservoir was low, and looked darker, more like brown. Maybe it's just hard to tell because it was low and it's a deep reservoir. The hose inside the reservoir, however, was wet for 2-3 inches from the bottom so I'm not too worried.

    Side note, I also noticed that the upper radiator hose has been replaced once before, as the NAPA sticker is still on it :)
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  4. NHerron

    NHerron Proven Member

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    No problem

    Yeah those coolant overflows aren't the most sealed. Anything that gets in just sinks to the bottom of that bottle for years and never goes anywhere.

    Welcome to Tuners by the way
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  5. DSM1G90

    DSM1G90 Supporting Member

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    You fill the radiator up to the neck of the filler unit, then fill the bottle up to the full line.

    To check, always check the radiator level, not the overflow level.
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  6. 19Eclipse90

    19Eclipse90 DSM Wiseman

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    Per the factory service manual, when the vehicle is cool, the coolant level should be at the FULL line; low and full level marks exist on the stock reservoirs so if that is what you have, it should be there: http://www.dsmtuners.com/forums/maintenance-repairs/430845-maintenance-guide.html.

    That'll help your hose inside the bottle remain submerged. ;)
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  7. motorking

    motorking Probationary Member

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    Hi,
    I work at Prestone coolants. We have alot of cooling system info on our website for new guys. You can use Prestone 50/50 antifreeze, it is compatipable with what is already in your car. If you cannot fing the level mark on your coolant recovery bottle, wait till the engine is cold, fill to the half way mark to be safe.
    Good luck!
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