overflow tank draws coolant back in?

5 replies to this topic
  • tool

Posted June 01, 2004 - 02:47 PM


i recently fitted a wr overflow tank on my yz. sweet mod. :thumbsup: i was wondering exactly how it draws coolant back into the radiator? a little coolant flowed into the tank but didnt go back into the rad. the rad level wasnt really low, but how does it get back in?

  • grayracer513

Posted June 01, 2004 - 07:56 PM


The way this works is fairly simple. As we know, the coolant expands when heated, and contracts when allowed to cool. The radiator cap is designed to accomodate both of these behaviors. Since the object is to retain the coolant, and to raise the boiling point by raising the operating pressure of the cooling system, the cap will only allow for expansion after a certain pressure is reached (about 16 psi on a standard 1.1 atmosphere cap). However, when the coolant cools down, the system needs to allow air to enter to replace any that escaped when the system pressure limit was reached. If this were not done, atmosheric pressure would flatten the radiator hoses, preventing circulation until the system heated back up considerably. For this purpose there is a small one-way check valve in the center of the sealing disc of the cap that allows air to enter to relieve any vacuum that might form as a result of the coolant contracting.

In order for this to become a coolant recovery system, there is one very important element - a seal at the top of the cap sealing the top of the radiator to the cap so that any air must enter the radiator through the vent hose. With this seal, and with the vent hose located down in the reservoir or expelled coolant, the contracting coolant will draw back fluid from the return can (which should be as near the height of the top of the raditor as is practical). Without it, air will simply enter at the top of the fill neck and the heavier coolant will not be pulled back to the radiator.

  • slappie

Posted June 02, 2004 - 02:32 AM



How did you mount the tank?

I've got all the parts sitting on my workbench but just haven't had the time to install them yet.

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  • tool

Posted June 02, 2004 - 10:30 AM


thanks for the info. where exactly is this check valve and how do i seal it? is it on top the cap or on bottom where the spring is? or should i just get a WR radiator cap?

and slappie, get some of that metal strapping that plumbers use to secure pipes. use that for the front mount and the rear fender bolt for the back. very solid :thumbsup:

  • grayracer513

Posted June 02, 2004 - 12:36 PM


The check valve doesn't need to be sealed, but if you remove the cap and look at the bottom, it's a small metal disk in the center of the rubber seal the holds pressure in the radiator. It's held shut with a weak spring, and you should be able to lift it easily with a fingernail.

What must be sealed in order for coolant recovery to work is the joint between the top edge of the radiator neck and the cap. The cap is intended to seal against the bottom of the neck to hold pressure in, but unless it was intended to be used in a recovery type system, there won't be any effort made to seal the top.

You need to look under the uppermost surface of the cap to see if there is a gasket there that would seal to the radiator neck's uppermost edge, above the vent tube. If none is there, you could make one out of some light gasket stock from any good auto supply store. If it's too thick, you won't be able to twist the cap back on all the way.

Some caps and radiators may fit together so well that they won't leak much air at this point when a vacuum forms inside the radiator. If the recovery tank is fairly high, it might work to some extent without an added seal, but not as well as if it has one.

  • tool

Posted June 02, 2004 - 02:55 PM


that makes sense and should be fairly easy. its funny how i didnt find anything about this in the searches i did. i knew it wasnt drawing coolant back in.
thanks for the help

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